A-Level Maths: Simplification and Operations of Algebraic Fractions | A-Level 数学：代数分式的化简与运算

📚 A-Level Maths: Simplification and Operations of Algebraic Fractions | A-Level 数学：代数分式的化简与运算

Algebraic fractions are fractions whose numerators and/or denominators are algebraic expressions. They appear throughout A-Level Mathematics, from pure mathematics to mechanics and statistics, and mastering their simplification and operations is essential for solving equations, inequalities, and calculus problems.

代数分式是指分子和/或分母为代数表达式的分数。它们贯穿 A-Level 数学的始终，从纯数学到力学与统计学，掌握其化简与运算是求解方程、不等式和微积分问题的基础。

1. Basic Concepts of Algebraic Fractions | 代数分式的基础概念

An algebraic fraction is a quotient of two algebraic expressions, such as (2x + 3)/(x² – 1). The numerator and denominator are polynomials. The domain of an algebraic fraction excludes values that make the denominator zero, as division by zero is undefined.

代数分式是两个代数表达式的商，例如 (2x + 3)/(x² – 1)。分子和分母都是多项式。分母为零的取值不在定义域内，因为除以零是没有意义的。

For example, the fraction 3x/(x – 2) is undefined when x = 2. When simplifying algebraic fractions, we must always bear the domain restriction in mind, especially when cancelling factors.

例如，分式 3x/(x – 2) 在 x = 2 时无定义。化简代数分式时，我们必须始终牢记定义域的限制，尤其是在约分的时候。

Domain: all real numbers except those making the denominator zero.
定义域：除使分母为零之外的所有实数。
Fractions with variables in the denominator require careful handling of restrictions.
分母中含有变量的分式需要谨慎处理限制条件。

2. Factorisation and Simplification | 因式分解与约分

The most fundamental technique for simplifying algebraic fractions is factorisation. By rewriting the numerator and denominator as products of factors, common factors can be identified and cancelled.

化简代数分式最基本的方法是因式分解。通过将分子和分母改写为因式的乘积，我们可以找出并约掉公因式。

Example: Simplify (x² – 4)/(x² – 2x). Factorise both parts: (x – 2)(x + 2) divided by x(x – 2). The common factor (x – 2) cancels, leaving (x + 2)/x, provided x ≠ 2 and x ≠ 0.

例：化简 (x² – 4)/(x² – 2x)。对分子分母分别因式分解：(x – 2)(x + 2) 除以 x(x – 2)。公因式 (x – 2) 约去，剩余 (x + 2)/x，前提是 x ≠ 2 且 x ≠ 0。

(x² – 4)/(x² – 2x) = (x – 2)(x + 2) / x(x – 2) = (x + 2)/x

Always factorise completely before cancelling.
约分之前务必完成因式分解。
Only factors that appear in both numerator and denominator can be cancelled.
只有分子分母共有的因式才能被约去。
State any restrictions on the variable after simplification.
化简后需说明变量的限制条件。

3. Multiplication of Algebraic Fractions | 代数分式的乘法

Multiplication of algebraic fractions follows the same rules as multiplication of numerical fractions: multiply numerators together and denominators together. Factorise first to simplify the process.

代数分式的乘法与数值分数的乘法规则相同：分子乘分子，分母乘分母。先因式分解可以简化计算过程。

Example: Multiply 2x/(x + 1) by (x + 1)²/(4x³). Factorise and cancel: the (x + 1) cancels partially, leaving 2x(x + 1)/(4x³) = (x + 1)/(2x²).

例：计算 2x/(x + 1) × (x + 1)²/(4x³)。因式分解并约分：(x + 1) 部分约去，得到 2x(x + 1)/(4x³) = (x + 1)/(2x²)。

2x/(x + 1) × (x + 1)²/(4x³) = (x + 1)/(2x²)

Factorise all numerators and denominators before multiplying.
相乘前先对所有分子分母进行因式分解。
Cancel common factors across the entire product.
在整个乘积中约去公因式。

4. Division of Algebraic Fractions | 代数分式的除法

Division of algebraic fractions is performed by multiplying by the reciprocal of the divisor. In other words, to divide by a fraction, we flip the divisor and change the division sign to multiplication.

代数分式的除法通过乘以除数的倒数来实现。换句话说，除以一个分式就是将被除数乘以翻转后的除数，并将除号改为乘号。

Example: Divide (x² – 9)/(x) by (x – 3)/(x²). First, rewrite: (x² – 9)/x ÷ (x – 3)/x² = (x² – 9)/x × x²/(x – 3). Factorise: (x – 3)(x + 3)/x × x²/(x – 3). Cancel (x – 3) and simplify: x(x + 3).

例：计算 (x² – 9)/x ÷ (x – 3)/x²。首先改写：(x² – 9)/x ÷ (x – 3)/x² = (x² – 9)/x × x²/(x – 3)。因式分解得 (x – 3)(x + 3)/x × x²/(x – 3)。约去 (x – 3) 并化简：x(x + 3)。

(x² – 9)/x ÷ (x – 3)/x² = x(x + 3), x ≠ 0, x ≠ 3

Flip the second fraction and multiply.
将第二个分式取倒数后相乘。
Remember to state domain restrictions from the original expression.
不要忘记根据原式写出定义域限制。

5. Addition and Subtraction of Algebraic Fractions | 代数分式的加减法

To add or subtract algebraic fractions, we need a common denominator. The most efficient approach is to find the lowest common multiple (LCM) of the denominators.

要进行代数分式的加减法，需要找到公分母。最高效的方法是求出分母的最小公倍式（LCM）。

Example: Simplify 2/(x + 1) + 3/(x – 2). The LCM of (x + 1) and (x – 2) is their product. Rewrite: 2(x – 2)/[(x + 1)(x – 2)] + 3(x + 1)/[(x + 1)(x – 2)] = [2(x – 2) + 3(x + 1)]/[(x + 1)(x – 2)] = (5x – 1)/[(x + 1)(x – 2)].

例：化简 2/(x + 1) + 3/(x – 2)。(x + 1) 和 (x – 2) 的最小公倍式就是它们的乘积。改写：2(x – 2)/[(x + 1)(x – 2)] + 3(x + 1)/[(x + 1)(x – 2)] = [2(x – 2) + 3(x + 1)]/[(x + 1)(x – 2)] = (5x – 1)/[(x + 1)(x – 2)]。

2/(x + 1) + 3/(x – 2) = (5x – 1)/((x + 1)(x – 2))

Find the LCM of the denominators first.
首先求出分母的最小公倍式。
Adjust each numerator by multiplying by the missing factor.
将每个分子乘以所缺的因式来调整。
Combine like terms in the final numerator.
在最终分子中合并同类项。

6. Complex Algebraic Fractions | 复杂代数分式的化简

A complex algebraic fraction has fractions within its numerator or denominator. These are often called composite fractions. The simplest way to simplify them is to multiply both the numerator and denominator of the larger fraction by the LCM of all the internal denominators.

复杂代数分式是指分子或分母中包含分式的分数，通常称为复合分式。化简它们的最简单方法是：用所有内部分母的最小公倍式同时乘以大分式的分子和分母。

Example: Simplify (1/x + 1)/(x – 1/x). The internal denominators are x. Multiply top and bottom by x: (1 + x)/(x² – 1) = (x + 1)/[(x – 1)(x + 1)] = 1/(x – 1), for x ≠ 0, x ≠ ±1.

例：化简 (1/x + 1)/(x – 1/x)。内部小分式的分母是 x。分子分母同时乘以 x：(1 + x)/(x² – 1) = (x + 1)/[(x – 1)(x + 1)] = 1/(x – 1)，其中 x ≠ 0，x ≠ ±1。

(1/x + 1)/(x – 1/x) = 1/(x – 1)

Multiply the whole complex fraction by the LCM of all inner denominators.
用所有内部分母的最小公倍式乘以整个复合分式。
Simplify the resulting expression carefully.
仔细化简所得表达式。
Check that no domain conditions are violated.
检查是否违反了定义域条件。

7. Improper Algebraic Fractions | 假分式的化简

When the degree of the numerator is greater than or equal to the degree of the denominator, the algebraic fraction is improper. Such fractions should be converted to the sum of a polynomial and a proper fraction using polynomial division.

当分子的次数大于或等于分母的次数时，代数分式称为假分式。这类分式应通过多项式除法转化为一个多项式与一个真分式之和。

Example: Simplify (x² + 3x + 2)/(x + 1). Performing division: (x² + 3x + 2) ÷ (x + 1) = x + 2, with no remainder. This is because x² + 3x + 2 = (x + 1)(x + 2).

例：化简 (x² + 3x + 2)/(x + 1)。进行除法：(x² + 3x + 2) ÷ (x + 1) = x + 2，没有余数。因为 x² + 3x + 2 = (x + 1)(x + 2)。

When the denominator does not divide the numerator exactly, the result is written as a polynomial plus a remainder fraction. For example:

当分母不能整除分子时，结果写成一个多项式加上一个余项分式。例如：

(x² + 3x + 1)/(x + 1) = x + 2 – 1/(x + 1)

Check degrees of numerator and denominator first.
先比较分子分母的次数。
Use algebraic long division or factorisation when possible.
使用长除法或尽可能因式分解。
The remainder must have degree strictly less than the divisor.
余项的次数必须严格低于除数。

8. Partial Fractions | 部分分式

Partial fractions is a reverse process: we decompose a single algebraic fraction into a sum of simpler fractions. This technique is especially useful in integration and binomial expansion.

部分分式是一个逆向过程：将一个代数分式分解为若干个更简单的分式之和。该技巧在积分和二项式展开中尤为有用。

For a proper fraction with distinct linear factors in the denominator, we write:

对于分母为互异一次因式的真分式，我们写成：

(3x + 5)/((x + 1)(x – 2)) = A/(x + 1) + B/(x – 2)

Multiply both sides by the common denominator: 3x + 5 = A(x – 2) + B(x + 1). Substitute x = 2 to eliminate A: 11 = 3B, so B = 11/3. Substitute x = -1 to eliminate B: 2 = -3A, so A = -2/3. Therefore:

两边同时乘以公分母：3x + 5 = A(x – 2) + B(x + 1)。代入 x = 2 消去 A：11 = 3B，因此 B = 11/3。代入 x = -1 消去 B：2 = -3A，因此 A = -2/3。因此：

(3x + 5)/((x + 1)(x – 2)) = -2/(3(x + 1)) + 11/(3(x – 2))

Distinct linear factors: assign constants A, B, C…
互异一次因式：设常数 A、B、C…
Repeated factors: assign A/(ax + b) + B/(ax + b)².
重复因式：设为 A/(ax + b) + B/(ax + b)²。
Quadratic factors: assign (Ax + B)/(cx² + dx + e).
二次因式：设为 (Ax + B)/(cx² + dx + e)。

9. Equations Involving Algebraic Fractions | 含代数分式的方程

Solving equations with algebraic fractions typically involves multiplying both sides of the equation by the LCM of the denominators to eliminate the fractions. This produces a polynomial equation that can be solved using standard methods.

解含代数分式的方程通常是在方程两边同时乘以分母的最小公倍式，以消去分式。这会得到一个可以用标准方法求解的多项式方程。

Example: Solve 2/x + 3/(x + 1) = 1. Multiply by x(x + 1): 2(x + 1) + 3x = x(x + 1). Simplify: 2x + 2 + 3x = x² + x. Rearrange: x² – 4x – 2 = 0. Using the quadratic formula:

例：解方程 2/x + 3/(x + 1) = 1。两边乘以 x(x + 1)：2(x + 1) + 3x = x(x + 1)。化简：2x + 2 + 3x = x² + x。移项：x² – 4x – 2 = 0。使用求根公式：

x = (4 ± √(16 + 8))/2 = 2 ± √6

Multiply every term by the LCM of all denominators.
用所有分母的最小公倍式乘以每一项。
Be aware that multiplying by an expression with variables may introduce extraneous roots.
注意：乘以含变量的表达式可能会引入增根。
Always check solutions against the original domain restrictions.
务必对照原始定义域检查解。

10. Common Mistakes and Exam Tips | 常见错误与备考建议

Students often make mistakes with algebraic fractions by cancelling terms instead of factors, or by misapplying the order of operations in complex fractions.

学生在处理代数分式时常犯的错误包括：把”项”当作”因式”来约分，或者在复杂分式中错误地使用运算顺序。

One common error is cancelling terms within a sum. For example, (x + 2)/(x + 3) cannot be simplified to 2/3, because x and 2 are terms, not factors. The fraction is already in its simplest form.

一个常见错误是在和式中直接约项。例如，(x + 2)/(x + 3) 不能化简为 2/3，因为 x 和 2 是”项”而非”因式”。该分式已是最简形式。

Only cancel multiplicative factors, never additive terms.
只能约去乘法意义上的因式，绝不能约去加法意义上的项。
Factorise every expression completely before simplifying.
化简前将每个表达式彻底因式分解。
Write down the domain restrictions at the start of your working.
在解题开始时先写下定义域限制。
In exam questions, ensure your final answer matches the required form.
在考试题目中，确保最终答案符合题设要求的形式。
Practice combining fractions and factorising quadratics until fluent.
反复练习合并分式和二次因式分解，直到熟练掌握。

By mastering these techniques, you will be well-prepared not only for questions directly about algebraic fractions, but also for calculus, coordinate geometry, and further mathematics topics where algebraic fractions regularly appear.

掌握了这些技巧，你不仅能从容应对直接考查代数分式的题目，还能为微积分、坐标几何以及其他频繁涉及代数分式的高等数学主题打下坚实基础。

Published by TutorHao | Mathematics Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

A-Level Maths: Simplification and Operations of Algebraic Fractions | A-Level 数学：代数分式的化简与运算