📚 Year 7 Cambridge Maths: High-Frequency Topics and Common Mistake Analysis | Year 7 Cambridge 数学：高频考点与易错题分析
Year 7 marks the start of the Cambridge Lower Secondary programme, where learners deepen their understanding of key mathematical concepts and develop problem-solving skills. This stage builds on primary maths and introduces more formal methods in number, algebra, geometry and data handling. Understanding which topics appear most often in assessments – and where students most commonly lose marks – can make a significant difference to both confidence and results. This article examines the high-frequency content areas in the Cambridge Year 7 maths syllabus and analyses typical errors, offering clear explanations and strategies to avoid them.
七年级是剑桥初中课程的起点，学生在数字、代数、几何和数据处理等方面深化理解并培养解题能力。此阶段在小学数学的基础上引入更规范的方法。了解哪些知识点在测评中出现频率最高、以及学生最容易在哪些地方失分，对于提升信心和成绩都至关重要。本文梳理了剑桥七年级数学的高频考点，并分析常见错误，提供清晰的解释和避坑策略。
1. Number Properties and Operations | 数的性质与运算
This topic revisits place value, factors, multiples, primes, squares, cubes and the order of operations. In Cambridge assessments, questions often combine several of these ideas. A high-frequency task is to identify the highest common factor (HCF) or lowest common multiple (LCM) of two numbers, or to use the BIDMAS/BODMAS rule correctly. Learners should also be confident working with large numbers and understanding index notation for squares and cubes.
本专题回顾位值、因数、倍数、质数、平方数、立方数以及运算顺序。在剑桥测评中，题目往往综合多个概念。高频任务包括求两个数的最大公因数（HCF）或最小公倍数（LCM），或者正确运用 BIDMAS/BODMAS 规则。学生还应熟练处理大数，并理解平方和立方的指数记法。
A classic mistake arises with the order of operations: for example, evaluating 3 + 4 × 2 as 14 instead of 11, because addition is performed before multiplication. Another common error involves prime numbers – some students still treat 1 as a prime, despite the definition requiring exactly two distinct factors. When working with squares and cubes, miscalculating 5² as 10 instead of 25 is a frequent slip. To avoid these, always write out the steps of BIDMAS (Brackets, Indices, Division/Multiplication, Addition/Subtraction) and double-check factor lists for HCF/LCM. Practice with negative bases, such as (-3)² = 9 while -3² = -9, to reinforce index rules.
典型的错误是运算顺序：例如，计算 3 + 4 × 2 时得到 14 而不是 11，因为先做了加法。另一个常见错误涉及质数——部分学生仍然把 1 当作质数，而定义要求恰好有两个不同的因数。处理平方和立方时，把 5² 误算为 10 而不是 25 也屡见不鲜。为避免这些错误，应当写出 BIDMAS（括号、指数、除/乘、加/减）的每一步，并在求 HCF/LCM 时复查因数列表。还要练习带负号底数的情况，如 (-3)² = 9，而 -3² = -9，以巩固指数规则。
2. Fractions, Decimals and Percentages | 分数、小数与百分数
Year 7 learners must fluently convert between fractions, decimals and percentages, perform all four operations with fractions, and solve percentage problems such as finding a percentage of a quantity. Cambridge exam questions frequently present a mixture of these forms in a single problem, requiring confident interchange. Common high-frequency tasks include simplifying fractions before multiplying, converting recurring decimals where applicable, and calculating percentage increase or decrease.
七年级学生需要熟练地在分数、小数和百分数之间转换，进行分数的加减乘除，并解决求一个数的百分之几等问题。剑桥试卷常在一道题中混合出现这些形式，要求灵活切换。高频任务包括：乘分数前先行化简，转换循环小数（如涉及），以及计算百分比增减。
An extremely common error is adding fractions without finding a common denominator, for example ½ + ⅓ = ⅖. Learners sometimes multiply denominators but forget to adjust numerators accordingly. In division, forgetting to multiply by the reciprocal (e.g. ¾ ÷ ½ computed as ¾ × ½) remains a recurring issue. With percentages, a typical blunder is thinking that a 20% increase followed by a 20% decrease returns to the original value – it does not, because the base changes. To improve accuracy, always rewrite problems step by step and check that the denominators match before adding. Convert percentages to decimals by dividing by 100, and use the decimal multiplier method for percentage change. Visual models like fraction walls can help reinforce the concept of equivalent fractions.
极为常见的错误是未找公分母就直接加分数，例如 ½ + ⅓ = ⅖。学生有时会乘分母但忘记相应调整分子。在除法中，忘记乘倒数（如 ¾ ÷ ½ 算成 ¾ × ½）也是反复出现的问题。百分数中，典型错误是认为先增加 20% 再减少 20% 会回到原数——事实并非如此，因为基数改变了。为提高准确性，应逐步写出解题过程，并在加法前检查分母是否一致。计算百分数时除以 100 化为小数，并使用小数乘数法处理百分比变化。分数墙等可视化模型有助于巩固等值分数的概念。
3. Algebraic Expressions and Equations | 代数表达式与方程
Algebra at this level focuses on writing expressions from word problems, collecting like terms, and solving simple linear equations. Cambridge Year 7 papers regularly ask students to simplify expressions such as 3a + 2b – a + 4b and to solve equations like 2x + 5 = 17. Building a strong foundation in algebraic notation and balance method is essential, as these skills underpin all future algebra work.
这一阶段的代数重点是从文字题写出表达式、合并同类项以及解简单的一元一次方程。剑桥七年级试卷常要求学生化简如 3a + 2b – a + 4b 的表达式，并解如 2x + 5 = 17 的方程。打下坚实的代数符号和平衡法基础至关重要，这些技能是未来所有代数学习的基石。
One of the most widespread mistakes is confusing the roles of letters and numbers when collecting like terms: for instance, writing 3x + 2y as 5xy. Students must remember that unlike terms cannot be combined. In solving equations, a frequent slip is applying an operation to only one side of the equation, or mishandling negative coefficients. For example, solving -x = 4 often yields x = 4 instead of x = -4. Another pitfall is the incorrect expansion of brackets, such as evaluating 2(x + 3) as 2x + 3. To overcome these, encourage using a ‘balance scale’ visualisation and always perform the same inverse operation on both sides. Practice writing the steps vertically: subtract 5, then divide by 2, and check the solution by substitution. Emphasise that 2(x + 3) means 2 × x + 2 × 3.
最普遍的错误之一是在合并同类项时混淆字母和数字的角色：例如，把 3x + 2y 写成 5xy。学生必须记住不同类项不能合并。解方程时，常见的失误是只对等式一边进行运算，或者处理负系数不当。例如，解 -x = 4 往往得到 x = 4 而不是 x = -4。另一个陷阱是错误展开括号，比如把 2(x + 3) 算成 2x + 3。为克服这些问题，可鼓励使用“天平”可视化，并始终在等式两边执行相同的逆运算。练习按步骤竖式书写：先减 5，再除以 2，并通过代入检验解。强调 2(x + 3) 表示 2 × x + 2 × 3。
4. Geometry: Angles and Shapes | 几何：角与图形
The geometry strand in Year 7 covers angle facts (angles on a straight line, around a point, vertically opposite angles), properties of triangles and quadrilaterals, and symmetry. Cambridge assessments expect learners to calculate missing angles using known facts and to classify shapes by their geometric properties. Accurate measurement and drawing of angles with a protractor is also tested, alongside line and rotational symmetry.
七年级几何涵盖角度知识（直线上的角、绕一点的角、对顶角）、三角形和四边形的性质以及对称。剑桥测评要求学生利用已知事实计算未知角度，并根据几何性质对图形进行分类。用量角器准确测量和绘制角度也是考点，线对称和旋转对称同样常见。
A high-frequency error is assuming that any two angles that look equal are equal, without applying formal rules. For example, many learners state that angles on a straight line add up to 180° but then miscalculate the complementary angle: if one is 73°, they might write the other as 117° by doing 180 – 73 incorrectly. Misidentifying types of triangles (e.g. calling a scalene triangle isosceles) or confusing rotational symmetry with line symmetry also cost marks. To avoid these, label known angles on the diagram, always write the angle fact being used (e.g. ‘angles on a straight line sum to 180°’), and double-check subtraction. When working with symmetry, trace the shape if needed and physically test how many ways it can be folded or rotated. Learning the specific properties of quadrilaterals – such as the diagonal properties of a rhombus – helps with classification questions.
高频错误是凭直觉认为两个看起来相等的角就相等，而不应用正式规则。例如，许多学生知道直线上的角之和为 180°，但在计算补角时出错：若一角为 73°，他们可能在 180 – 73 时算错而得出 117°。错误辨认三角形类型（如将不等边三角形说成等腰），或把旋转对称与线对称混淆，也会导致失分。为避免这些错误，应在图上标记已知角度，始终写出所用的角度事实（例如“直线上的角总和为 180°”），并复查减法。处理对称时，可以描摹图形，实际测试折叠或旋转的方式。学习四边形的特殊性质——如菱形的对角线性质——有助于分类题。
5. Measurement and Units | 测量与单位
Measurement in Year 7 involves perimeter, area of rectangles and compound shapes, volume of cuboids, and conversions between metric units. Cambridge papers frequently pose problems where lengths are given in different units, requiring conversion before calculation. Time calculations – such as finding the difference between times or adding durations – also appear regularly.
七年级的测量部分包括周长、矩形和组合图形的面积、长方体的体积以及公制单位换算。剑桥试卷常出现长度单位不一致的题目，需要先换算再计算。时间计算——比如求时间差或加时长——也经常出现。
Typical mistakes include confusing perimeter and area formulas, especially when a shape is not a simple rectangle. Many learners multiply all side lengths to find area, even for a rectangle, forgetting that area = length × width requires only two dimensions. In unit conversions, a common slip is moving the decimal point in the wrong direction: for example, converting 5.2 m to cm as 5200 cm instead of 520 cm. With volume, students sometimes use area formulas or miscount the number of cubes. For time problems, crossing an hour boundary often leads to errors, such as calculating the duration from 09:45 to 10:15 as 1 hour 30 minutes. To improve, draw and label diagrams with dimensions, underline the unit required in the answer, and use number lines for time intervals. Practise common conversion facts: 1 km = 1000 m, 1 m = 100 cm, 1 cm = 10 mm, 1 kg = 1000 g, 1 litre = 1000 ml. The ‘multiply or divide by powers of 10’ rule must become automatic.
典型错误包括混淆周长和面积公式，尤其是在处理非简单矩形时。许多学生会用所有边长相乘来求面积，即便对于矩形也如此，而忘记了面积 = 长 × 宽 只需要两个维度。单位换算中，常见失误是小数点点错方向：例如，将 5.2 m 换算成 5200 cm 而不是 520 cm。关于体积，学生有时使用面积公式或错数立方体个数。时间问题中，跨越整点边界常导致错误，比如计算 09:45 到 10:15 的时长得到 1小时30分钟。为改进，应绘制并标注尺寸图，在答案处划线强调所需单位，并利用时间线处理时间间隔。练习常见的换算关系：1 km = 1000 m，1 m = 100 cm，1 cm = 10 mm，1 kg = 1000 g，1 litre = 1000 ml。乘以或除以 10 的幂的规则必须达到自动化程度。
6. Data Handling and Statistics | 数据处理与统计
In Year 7, students learn to calculate averages (mean, median, mode) and the range, and to interpret bar charts, pictograms, line graphs and pie charts. Cambridge exam questions often require learners to compare two data sets using the mean and range, or to extract information from a chart to construct a frequency table. Understanding what each average represents and choosing the most appropriate one is a key skill.
在七年级，学生学习计算平均数（均值、中位数、众数）和极差，并解读条形图、象形图、线形图和饼图。剑桥试题常要求学生利用均值和极差比较两组数据，或从图表中提取信息来构建频数表。理解每个平均数的含义并选择最合适的一个是关键技能。
A common mistake is confusing the three averages: using the largest value instead of the mode, or forgetting to order the data before finding the median. When finding the median of an even‑numbered set, many students pick the middle number without averaging the two middle values. In calculating the mean, a frequent error is dividing by the number of categories rather than the total number of data points. When interpreting pie charts, pupils may misread the sector angle or treat the angle as the frequency. Graphical errors include not reading the scale on a bar chart correctly, especially when one division represents more than one unit. To address these, teach a clear routine: for median – order, count, locate; for mean – sum, count, divide. With charts, always check the axis labels and scale before answering; highlight them on the paper. Constructing a frequency table from raw data helps to organise thinking.
常见错误是混淆三种平均数：用最大值代替众数，或求中位数前忘记排序。对偶数个数据的中位数，许多学生直接取中间位置而忘记对中间两个数求均值。计算均值时，常犯的错误是除以类别数而非数据点总数。解读饼图时，可能误读扇形角度或把角度当作频数。图形方面的错误包括未正确读取条形图上的刻度，尤其是当一格代表多个单位时。为解决这些问题，应教给学生清晰的流程：中位数——排序、计数、定位；均值——求和、计数、相除。看图时必须先检查坐标轴标签和刻度；可在卷面上勾画强调。根据原始数据构建频数表有助于理顺思路。
7. Ratio and Proportion | 比与比例
Ratio and proportion work at this level includes simplifying ratios, sharing a quantity in a given ratio, and using simple map scales. Cambridge assessments often link ratio to fractions and real-life contexts such as recipes or mixtures. The notation a:b is used, and learners must recognise that the order of terms is crucial.
这一阶段的比与比例包括化简比、按给定比例分配数量以及使用简单地图比例尺。剑桥测评常将比与分数和现实情境（如食谱或混合物）联系起来。使用 a:b 记法，并且学生必须认识到各项的顺序至关重要。
High-impact errors include reversing the order of a ratio: for example, ‘the ratio of boys to girls is 3:2’ means for every 3 boys there are 2 girls, but some learners write the opposite. When sharing £50 between two people in the ratio 3:2, a typical mistake is to divide £50 by 2 and then multiply, rather than dividing by the total number of parts (3+2=5) first. Unit mismatches also cause problems – if a ratio compares 2 m to 50 cm, both quantities must be in the same unit before simplifying. To avoid confusion, encourage underlining the key words to fix the order, and always calculate the total number of parts. Draw a bar model to visualise the sharing. When simplifying, divide both terms by their HCF and make sure both quantities have the same unit.
高发错误包括颠倒比的顺序：例如，“男孩与女孩的比是 3:2” 意味着每 3 个男孩就有 2 个女孩，但一些学生会反过来写。当按 3:2 的比例分配 £50 给两人时，典型错误是先将 £50 除以 2 再乘，而正确的做法是先除以总份数 (3+2=5)。单位不一致也会导致问题——如果比比较 2 m 与 50 cm，则必须先化为相同单位再化简。为免混淆，鼓励圈划关键词以固定顺序，并始终先求总份数。绘制条形模型直观展示分配过程。化简时，用最大公因数除以两项，并确保两个量单位相同。
8. Sequences and Patterns | 数列与规律
Year 7 sequence work focuses on recognising and continuing linear patterns, both in numbers and in matchstick-style diagrams. Learners begin to describe the term-to-term rule and, in some cases, find a simple position-to-term formula (nth term) for a linear sequence. Cambridge tasks may ask for the next few terms, a specific term like the 10th, or the rule in words.
七年级的数列主题着重于识别并延续线性模式，包括数字序列和火柴棒式图形。学生开始描述项到项的规则，并在某些情况下找出线性数列的简易位置-项公式（第 n 项）。剑桥试题可能要求写出紧接着的几项、某一项（如第 10 项），或用文字描述规律。
A persistent mistake is confusing the position number with the term value. When asked for the 5th term of the sequence 3, 7, 11, 15,…, learners may simply write 5 instead of 19. Others misidentify the pattern, especially when the increase is not constant because they have misread the numbers. In diagrammatic sequences, students may count the number of items incorrectly, missing hidden matches or extra edges. A powerful strategy is to write the sequence with term numbers above: 1→3, 2→7, 3→11, 4→15, clearly showing the difference of +4 each time. Then extend logically: the nth term rule can be expressed as 4n – 1. Always check the rule by testing with n=1,2. Encourage learners to verbalise the pattern: ‘start at 3 and add 4 each time’ before answering. Drawing the next diagram step by step reduces counting errors.
持续出现的错误是把位置序号与项值混淆。当要求写出数列 3, 7, 11, 15,… 的第 5 项时，学生可能直接写 5 而不是 19。另有一些人则错误辨识规律，特别是当增量不恒定时，因为他们误读了数字。在图形序列中，学生可能数错物件数量，漏掉隐藏的火柴棒或多余的边。一个有效策略是在各项上标出位置序号：1→3，2→7，3→11，4→15，清晰显示每次增加 +4。然后逻辑延伸：第 n 项规则可表达为 4n – 1。务必用 n=1,2 检验规则。鼓励学生在回答前口头描述规律：“从 3 开始，每次加 4”。逐步画下一个图形可减少计数错误。
9. Negative Numbers and the Number Line | 负数与数轴
Working with negative numbers is a key Year 7 topic. Students must order, add, subtract, multiply and divide negative integers, and apply these skills in contexts such as temperature or bank balances. Cambridge questions regularly involve calculations like -5 + 3, 2 – (-4), or (-3) × (-6), often embedded within multi-step problems.
负数的运算是七年级的重要内容。学生必须会排序、加、减、乘、除负整数，并将这些技能应用于温度或银行账户余额等情境。剑桥试题经常包含 -5 + 3、2 – (-4) 或 (-3) × (-6) 这类计算，且常嵌入多步问题中。
The most notorious error is mishandling subtraction of a negative: many pupils treat 2 – (-4) as 2 – 4 = -2, instead of 2 + 4 = 6. This shows a misunderstanding of the double-negative rule. Addition with mixed signs also trips up learners; for -5 + 3, some answer -8, while the correct answer is -2. In multiplication and division, the sign rules ‘same sign positive, different signs negative’ are often memorised but incorrectly applied to addition. A useful model is the number line: for addition, start at the first number and move right for positive, left for negative. For subtraction, add the opposite. Drill quick mental exercises daily: “What is -3 – 5?”, “What is -4 × -2?”. Encourage writing the intermediate step: 2 – (-4) = 2 + 4 = 6. Reinforce that multiplying or dividing two negatives gives a positive, and this is different from adding negatives.
最为人熟知的错误是减去负数的处理：许多学生把 2 – (-4) 当作 2 – 4 = -2，而不是正确的 2 + 4 = 6。这反映出对双重负号规则的理解不足。异号相加也常绊倒学生；对于 -5 + 3，有些人回答 -8，而正确答案是 -2。乘除法中，“同号得正，异号得负”的符号法则常被记住却错误用于加法。一个有用的模型是数轴：加法从第一个数出发，正向右，负向左。减法则是加上相反数。每天进行快速心算练习：“-3 – 5 是多少？”“-4 × -2 是多少？”。鼓励写出中间步骤：2 – (-4) = 2 + 4 = 6。强化两个负数相乘或相除得正，这与负数相加不同。
10. Probability Basics | 概率基础
In Year 7, probability is introduced on a scale from 0 to 1, including words like impossible, unlikely, even chance, likely, certain. Learners calculate simple probabilities from equally likely outcomes and list all possible outcomes systematically. Cambridge tasks often involve rolling dice, spinning spinners, or picking coloured counters from a bag.
七年级介绍了从 0 到 1 的概率尺度，包括不可能、不太可能、对等机会、很可能、肯定等词语。学生需根据等可能结果计算简单概率，并系统地列出所有可能结果。剑桥试题常涉及掷骰子、转旋转盘或从袋中取彩色筹码。
A fundamental mistake is writing a probability as a number greater than 1, such as 5/3, showing confusion between ‘number of favourable outcomes’ and ‘total outcomes’. Learners also forget to simplify fractions, or they leave answers as ‘1 out of 4’ instead of ¼. When two events are combined, some students double-count or miss outcomes; for example, when rolling two dice, they may not list all 36 possibilities systematically. Using a sample space diagram or a simple table reduces this oversight. In ‘expected number of times’ problems, the common slip is to multiply the probability by the number of trials incorrectly, or to round without considering the context. Stress that probability = (number of favourable outcomes) / (total number of outcomes), and the total must be the denominator. Always check that the final fraction is between 0 and 1. Use listing strategies like ordered pairs for two spinners.
一个基本错误是把概率写成大于 1 的数，如 5/3，这显示出混淆了“有利结果数”和“总结果数”。学生还不记得化简分数，或者把答案写成“4 个中的 1 个”而不是 ¼。当两个事件组合时，有些学生会重复计数或遗漏结果；例如，掷两个骰子时，可能没有系统地列出全部 36 种可能。使用样本空间图或简单表格可减少这种疏漏。在“预期次数”问题中，常见失误是错误地将概率乘以试验次数，或不考虑情境就四舍五入。强调概率 = (有利结果数) / (总结果数)，且总结果数必须为分母。始终检查最终分数是否介于 0 与 1 之间。对于两个转盘，采用有序数对的列举策略。
11. Problem-Solving and Mathematical Reasoning | 问题解决与数学推理
Many Cambridge exam questions are set in unfamiliar contexts to test reasoning. In Year 7, this could be a multi-step word problem involving money, measures, or logic puzzles. High achievers are expected to break down complex problems, identify relevant information, and choose appropriate operations without being told which one to use. These questions often combine several topics.
剑桥试题中有许多题目设置在不熟悉的情境中，以测试推理能力。在七年级，这可能是一道涉及金钱、度量或逻辑谜题的多步文字题。高分段学生应能拆解复杂问题，识别相关信息，并自行选择合适的运算而不需提示。这些题目通常综合多个专题。
Common pitfalls include rushing to calculate before fully understanding the question, and giving up when a problem looks unfamiliar. Learners often apply the most recent method they learned, rather than thinking about what the problem requires. For instance, seeing the word ‘more’ might lead them to add, when subtraction is needed. In multi-step problems, a typical error is forgetting to carry over a result, or not reading the final question carefully – answering an intermediate step instead of what was actually asked. To improve, teach the ‘RUCSAC’ strategy (Read, Understand, Choose, Solve, Answer, Check). Underline key numbers and words, annotate the question, and draw bar models or diagrams. Practise ‘explain your reasoning’ prompts, as they develop a habit of justifying steps. Regularly expose students to non-routine problems to build resilience.
常见的陷阱包括在完全理解问题之前就匆忙计算，或者当问题看起来陌生时就放弃。学生往往套用最近学过的方法，而不思考问题究竟需要什么。例如，看到“多”一词就可能做加法，但实际需要减法。在多步问题中，典型错误是忘记代入上一步的结果，或未仔细阅读最后的提问——回答的是中间步骤而不是实际所问。为改进，教授“RUCSAC”策略（读题、理解、选择、解答、回答、检查）。圈划关键数字和词语，给题目加上注释，并绘制条形模型或示意图。练习“解释你的推理”这类提示，因为它们能培养说明步骤的习惯。定期让学生接触非常规问题，以建立韧性。
12. Revision and Exam Technique | 复习与考试技巧
Securing top marks in Cambridge Year 7 maths tests is not only about knowing the content; it also depends on effective revision habits and examination technique. Many capable students lose marks unnecessarily through poor presentation, missing units, or rushing through easy sections. A structured approach to revision and timed practice can dramatically improve performance.
在剑桥七年级数学测评中取得高分，不仅取决于知识掌握，还有赖于高效的复习习惯和应试技巧。很多能力不错的学生由于版面凌乱、遗漏单位或匆忙完成简单部分而白白丢分。有条理的复习和限时练习可显著提升表现。
Frequent technique errors include not showing working, which is particularly damaging when part marks are available for method, even if the final answer is wrong. Some learners leave units off their answer, or round incorrectly in measurement problems. Another issue is poor time management – spending too long on a challenging problem and leaving no time to check. In non-calculator papers, simple arithmetic mistakes like 8 × 7 = 54 can cost several marks. To counter these, always show your working step by step; number each step clearly. Practise writing answers with correct units: for area, cm²; for volume, cm³. Do timed past papers in exam conditions, then use a mark scheme to identify recurring careless errors. Maintain a ‘mistakes log’ to record typical slips and review it before the test. Finally, adopt a question‑reading routine: read twice, highlight key instructions, and check the answer against the question before moving on.
常见的作答技巧错误包括不展示计算过程，这在有方法分的题目中尤为不利，即使最终答案错误仍可得分。一些学生忘记写单位，或在测量题中四舍五入不当。另一个问题是时间管理——在难题上耗费太久而没时间检查。在非计算器卷中，像 8 × 7 = 54 这样的简单计算错误可能丢掉好几分。为应对这些问题，务必逐步写出演算过程；每一步都清楚编号。练习书写带正确单位的答案：面积用 cm²，体积用 cm³。在考试条件下定时做历年试卷，然后对照评分方案找出反复出现的粗心错误。建立“错题日志”记录典型失误，考前复习。最后，形成读题流程：读两遍，高亮关键指令，在进入下一题前将答案与问题核对一遍。
Published by TutorHao | Mathematics Revision Series | aleveler.com
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