The Scenario
You know that $\dfrac{1}{2} + \dfrac{1}{3} = \dfrac{5}{6}$. Combining two fractions into one is easy — find a common denominator and add. But now the question turns around: if I hand you $\dfrac{5}{6}$ and ask "split this into two unit fractions," can you recover $\dfrac{1}{2} + \dfrac{1}{3}$?
The Question
You know $\dfrac{1}{x-2} + \dfrac{1}{x+3} = \dfrac{2x+1}{(x-2)(x+3)}$. But if I give you $\dfrac{5}{(x-2)(x+3)}$, how would you split it back into two simpler fractions — without guessing?
Green (solid): target original $\dfrac{2x^2+5x-1}{(x-1)(x+1)(x+2)}$. Amber (dashed): current partial sum. Adjust $A$, $B$, $C$ until the curves match.
A repeated factor $(cx+d)^2$ gets two partial fractions: one with denominator $(cx+d)$ and one with $(cx+d)^2$.
Green (solid): target original $\dfrac{3x^2+3x+2}{(x-1)(x+1)^2}$. Amber (dashed): current partial sum. Adjust $A$, $B$, $C$ until the curves match.
Group work (5 min): In pairs, match each fraction to its correct decomposition form. Click a fraction card, then click the matching form card. Race to get all 4 correct!
If $\dfrac{2x^2+5x-1}{(x-1)(x+1)(x+2)} = \dfrac{A}{x-1} + \dfrac{B}{x+1} + \dfrac{C}{x+2}$, what happens when you multiply both sides by the full denominator $(x-1)(x+1)(x+2)$ and then set $x = 1$? Think first.
Think-Pair-Share (5 min): Follow the 4-step interactive guide to discover how multiplying through by the full denominator — then substituting a well-chosen root — isolates each constant. After isolating one constant, discuss with your partner how to find the remaining constants.
Write $\dfrac{6x^2 + 22x + 18}{(x + 1)(x + 2)(x + 3)}$ as a sum of partial fractions.
Three distinct linear factors, so use $\dfrac{A}{x+1} + \dfrac{B}{x+2} + \dfrac{C}{x+3}$. Multiply through by the full denominator to form an identity, then substitute the roots $x = -1$, $-2$, $-3$.
$$\dfrac{6x^2 + 22x + 18}{(x+1)(x+2)(x+3)} = \dfrac{A}{x+1} + \dfrac{B}{x+2} + \dfrac{C}{x+3}$$
Multiplying both sides by $(x+1)(x+2)(x+3)$ clears every fraction and gives a polynomial identity:
$$6x^2 + 22x + 18 \equiv A(x+2)(x+3) + B(x+1)(x+3) + C(x+1)(x+2)$$
Find $A$: set $x = -1$ (the root of $x+1$) — the $B$ and $C$ terms vanish:
$$6(-1)^2 + 22(-1) + 18 = A(-1+2)(-1+3) \;\Rightarrow\; 2 = 2A \;\Rightarrow\; A = 1$$
Find $B$: set $x = -2$ (the root of $x+2$):
$$6(-2)^2 + 22(-2) + 18 = B(-2+1)(-2+3) \;\Rightarrow\; -2 = -B \;\Rightarrow\; B = 2$$
Find $C$: set $x = -3$ (the root of $x+3$):
$$6(-3)^2 + 22(-3) + 18 = C(-3+1)(-3+2) \;\Rightarrow\; 6 = 2C \;\Rightarrow\; C = 3$$
Once fluent, you can compress each substitution: "cover up" the factor $(x+1)$ in the original denominator and substitute $x = -1$ into what remains — $A = \dfrac{6(1) + 22(-1) + 18}{(-1+2)(-1+3)} = 1$, as above. Useful for checking, but in written CIE solutions show the full identity and your substitutions.
$\dfrac{1}{x+1} + \dfrac{2}{x+2} + \dfrac{3}{x+3} = \dfrac{(x+2)(x+3) + 2(x+1)(x+3) + 3(x+1)(x+2)}{(x+1)(x+2)(x+3)}$
Numerator $= (x^2+5x+6) + 2(x^2+4x+3) + 3(x^2+3x+2) = 6x^2 + 22x + 18$. ✓
$$\dfrac{6x^2 + 22x + 18}{(x+1)(x+2)(x+3)} = \dfrac{1}{x+1} + \dfrac{2}{x+2} + \dfrac{3}{x+3}$$
Express $\dfrac{4}{(1 - 3x)(1 - x)^2}$ as a sum of partial fractions.
The factor $(1-x)^2$ is repeated, so it needs two terms: $\dfrac{B}{1-x} + \dfrac{C}{(1-x)^2}$. Multiply through by the full denominator to form an identity, substitute $x = \dfrac{1}{3}$ and $x = 1$ to find $A$ and $C$, then a convenient value (e.g. $x = 0$) for $B$.
$$\dfrac{4}{(1-3x)(1-x)^2} = \dfrac{A}{1-3x} + \dfrac{B}{1-x} + \dfrac{C}{(1-x)^2}$$
Multiply both sides by $(1-3x)(1-x)^2$ to clear all fractions:
$$4 \equiv A(1-x)^2 + B(1-3x)(1-x) + C(1-3x)$$
Find $A$: set $x = \dfrac{1}{3}$ (the root of $1-3x$) — the $B$ and $C$ terms vanish:
$$4 = A\left(1 - \dfrac{1}{3}\right)^2 = A\left(\dfrac{2}{3}\right)^2 = \dfrac{4A}{9} \;\Rightarrow\; A = 9$$
Find $C$: set $x = 1$ (the root of $1-x$):
$$4 = C(1 - 3) = -2C \;\Rightarrow\; C = -2$$
Find $B$: substitute $x = 0$ (a convenient value) into the identity:
$$4 = A(1) + B(1)(1) + C(1) = 9 + B - 2 \;\Rightarrow\; B = -3$$
Multiply both sides by $(1-3x)(1-x)^2$:
$$4 = A(1-x)^2 + B(1-3x)(1-x) + C(1-3x)$$
Expand:
$$\begin{aligned} 4 &= A(1 - 2x + x^2) + B(1 - 4x + 3x^2) + C(1 - 3x) \\ &= (A + 3B)x^2 + (-2A - 4B - 3C)x + (A + B + C) \end{aligned}$$
Equate coefficients of corresponding powers of $x$:
$$\begin{cases} x^2: & A + 3B = 0 \\ x^1: & -2A - 4B - 3C = 0 \\ x^0: & A + B + C = 4 \end{cases}$$
From $x^2$: $A = -3B$. Substitute into $x^0$: $-3B + B + C = 4 \;\Rightarrow\; C = 4 + 2B$.
Substitute into $x^1$: $-2(-3B) - 4B - 3(4+2B) = 0 \;\Rightarrow\; 6B - 4B - 12 - 6B = 0 \;\Rightarrow\; -4B = 12 \;\Rightarrow\; B = -3$.
Then $A = -3(-3) = 9$, $C = 4 + 2(-3) = -2$.
At $x = -1$: LHS $= \dfrac{4}{(4)(4)} = \dfrac{1}{4}$. RHS $= \dfrac{9}{4} - \dfrac{3}{2} - \dfrac{2}{4} = \dfrac{9-6-2}{4} = \dfrac{1}{4}$. ✓
$$\dfrac{4}{(1-3x)(1-x)^2} = \dfrac{9}{1-3x} - \dfrac{3}{1-x} - \dfrac{2}{(1-x)^2}$$
Let $\text{f}(x) \equiv \dfrac{x^2 + 3x + 3}{(x + 1)(x + 3)}$. Express $\text{f}(x)$ in partial fractions.
The numerator and denominator both have degree 2. Expand the denominator, then perform polynomial division to separate the whole part from the proper fraction.
Expand the denominator: $(x+1)(x+3) = x^2 + 4x + 3$.
Since $\deg(\text{numerator}) = \deg(\text{denominator}) = 2$, the fraction is improper. Perform long division:
So $\text{f}(x) = 1 + \dfrac{-x}{(x+1)(x+3)}$.
$$\dfrac{-x}{(x+1)(x+3)} = \dfrac{A}{x+1} + \dfrac{B}{x+3}$$
Multiplying both sides by $(x+1)(x+3)$ gives the identity $-x \equiv A(x+3) + B(x+1)$.
Find $A$: set $x = -1$ (the root of $x+1$): $1 = 2A \;\Rightarrow\; A = \dfrac{1}{2}$.
Find $B$: set $x = -3$ (the root of $x+3$): $3 = -2B \;\Rightarrow\; B = -\dfrac{3}{2}$.
At $x = 0$: LHS $= \dfrac{3}{3} = 1$. RHS $= 1 + \dfrac{1/2}{1} - \dfrac{3/2}{3} = 1 + \dfrac{1}{2} - \dfrac{1}{2} = 1$. ✓
$$\text{f}(x) = 1 + \dfrac{1}{2(x+1)} - \dfrac{3}{2(x+3)}$$
Test your understanding of all four lesson objectives. Each question targets one objective — try each before moving on!
Which is the correct form for decomposing $\dfrac{P(x)}{(x-2)(x+1)^2}$?
A repeated factor $(x+1)^2$ needs two partial fractions: one with denominator $(x+1)$ and one with $(x+1)^2$. The form $\dfrac{A}{x-2} + \dfrac{B}{x+1} + \dfrac{C}{(x+1)^2}$ covers all factors of the denominator.
Multiplying $\dfrac{x+5}{(x-1)(x+2)} = \dfrac{A}{x-1} + \dfrac{B}{x+2}$ through by $(x-1)(x+2)$ gives the identity $x + 5 \equiv A(x+2) + B(x-1)$. Which value of $x$ finds $A$ in one step?
Substituting the root of the factor $(x-1)$ — that is, $x = 1$ — makes the $B(x-1)$ term vanish: $1 + 5 = A(1+2)$, so $A = 2$. In general, substitute the root of each linear factor into the identity to isolate its constant. (The cover-up shortcut is a compressed mental version of exactly this substitution.)
In $\dfrac{3x}{(2x-1)(x+1)} = \dfrac{A}{2x-1} + \dfrac{B}{x+1}$, multiplying through gives the identity $3x \equiv A(x+1) + B(2x-1)$. What value of $x$ should you substitute to find $A$?
The factor $(2x-1)$ has root $x = \dfrac{1}{2}$ (solving $2x - 1 = 0$). Substituting $x = \dfrac{1}{2}$ into the identity makes the $B(2x-1)$ term vanish, leaving $\dfrac{3}{2} = A \cdot \dfrac{3}{2}$, so $A = 1$. When the factor is $(ax+b)$ with $a \neq 1$, the root is $x = -\dfrac{b}{a}$, not $x = -b$.
In $\dfrac{P(x)}{(x-1)(x+2)^2} = \dfrac{A}{x-1} + \dfrac{B}{x+2} + \dfrac{C}{(x+2)^2}$, how do you find $B$?
Multiplying through gives $P(x) \equiv A(x+2)^2 + B(x-1)(x+2) + C(x-1)$. Substituting $x = 1$ isolates $A$; substituting $x = -2$ isolates $C$. But no single root isolates $B$ — its term $(x-1)(x+2)$ shares the factor $(x+2)$ with the $C$ term's $(x+2)^2$. So once $A$ and $C$ are known, substitute any other convenient value (e.g. $x = 0$) into the identity and solve for $B$.
What must you do first when decomposing $\dfrac{x^2 + 3x + 3}{(x+1)(x+3)}$ into partial fractions?
The numerator has degree 2 and the denominator expands to degree 2, so the fraction is improper. You must perform polynomial division first to obtain $Q(x) + \dfrac{R(x)}{D(x)}$ where $\deg(R) < \deg(D)$. Only then can you decompose the proper fraction.
Write $\dfrac{x + 5}{(x - 1)(x + 2)}$ as a sum of partial fractions.
$$\dfrac{x+5}{(x-1)(x+2)} = \dfrac{A}{x-1} + \dfrac{B}{x+2}$$
Multiplying through by $(x-1)(x+2)$ gives the identity $x + 5 \equiv A(x+2) + B(x-1)$.
Find $A$: set $x = 1$ (root of $x-1$): $6 = 3A \;\Rightarrow\; A = 2$.
Find $B$: set $x = -2$ (root of $x+2$): $3 = -3B \;\Rightarrow\; B = -1$.
$\dfrac{2}{x-1} - \dfrac{1}{x+2} = \dfrac{2(x+2)-(x-1)}{(x-1)(x+2)} = \dfrac{x+5}{(x-1)(x+2)}$. ✓
$$\dfrac{x+5}{(x-1)(x+2)} = \dfrac{2}{x-1} - \dfrac{1}{x+2}$$
Express $\dfrac{4x^2 - 3}{x(2x - 1)^2}$ as a sum of partial fractions.
The denominator has a distinct factor $x$ and a repeated factor $(2x-1)^2$. The form is $\dfrac{A}{x} + \dfrac{B}{2x-1} + \dfrac{C}{(2x-1)^2}$. Multiply through by the full denominator to form an identity, substitute the roots $x = 0$ and $x = \dfrac{1}{2}$ to find $A$ and $C$, then a convenient value (e.g. $x = 1$) for $B$.
$$\dfrac{4x^2-3}{x(2x-1)^2} = \dfrac{A}{x} + \dfrac{B}{2x-1} + \dfrac{C}{(2x-1)^2}$$
Multiplying through by $x(2x-1)^2$ gives the identity $4x^2 - 3 \equiv A(2x-1)^2 + Bx(2x-1) + Cx$.
Find $A$: set $x = 0$ (root of $x$): $-3 = A(-1)^2 \;\Rightarrow\; A = -3$.
Find $C$: set $x = \dfrac{1}{2}$ (root of $2x-1$): $1 - 3 = \dfrac{C}{2} \;\Rightarrow\; C = -4$.
Find $B$: substitute $x = 1$ into the identity:
$$4 - 3 = A(1) + B(1)(1) + C(1) \;\Rightarrow\; 1 = -3 + B - 4 \;\Rightarrow\; B = 8$$
At $x = 2$: LHS $= \dfrac{16-3}{2 \cdot 9} = \dfrac{13}{18}$. RHS $= \dfrac{-3}{2} + \dfrac{8}{3} - \dfrac{4}{9} = \dfrac{-27+48-8}{18} = \dfrac{13}{18}$. ✓
$$\dfrac{4x^2-3}{x(2x-1)^2} = -\dfrac{3}{x} + \dfrac{8}{2x-1} - \dfrac{4}{(2x-1)^2}$$
Write $\dfrac{4x^2 - 25x - 3}{(2x + 1)(x - 1)(x - 3)}$ as a sum of partial fractions.
Three distinct linear factors including $(2x+1)$. The root of $(2x+1)$ is $x = -\frac{1}{2}$. Multiply through by the full denominator to form an identity, then substitute each root $x = -\frac{1}{2}$, $1$, $3$ — each substitution kills two terms at once.
$$\dfrac{4x^2-25x-3}{(2x+1)(x-1)(x-3)} = \dfrac{A}{2x+1} + \dfrac{B}{x-1} + \dfrac{C}{x-3}$$
Multiplying through by $(2x+1)(x-1)(x-3)$ gives the identity:
$$4x^2 - 25x - 3 \equiv A(x-1)(x-3) + B(2x+1)(x-3) + C(2x+1)(x-1)$$
Find $A$: set $x = -\dfrac{1}{2}$ (root of $2x+1$):
$$4\cdot\tfrac{1}{4} + \tfrac{25}{2} - 3 = A\left(-\tfrac{3}{2}\right)\left(-\tfrac{7}{2}\right) \;\Rightarrow\; \tfrac{21}{2} = \tfrac{21}{4}A \;\Rightarrow\; A = 2$$
Find $B$: set $x = 1$ (root of $x-1$): $4 - 25 - 3 = B(3)(-2) \;\Rightarrow\; -24 = -6B \;\Rightarrow\; B = 4$.
Find $C$: set $x = 3$ (root of $x-3$): $36 - 75 - 3 = C(7)(2) \;\Rightarrow\; -42 = 14C \;\Rightarrow\; C = -3$.
Numerator of RHS $= 2(x-1)(x-3) + 4(2x+1)(x-3) - 3(2x+1)(x-1)$
$= (2x^2-8x+6) + (8x^2-20x-12) - (6x^2-3x-3) = 4x^2 - 25x - 3$. ✓
$$\dfrac{4x^2-25x-3}{(2x+1)(x-1)(x-3)} = \dfrac{2}{2x+1} + \dfrac{4}{x-1} - \dfrac{3}{x-3}$$
Find the values of the constants $A$, $B$, $C$ and $D$ such that $$\dfrac{2x^3 - 1}{x^2(2x - 1)} \equiv A + \dfrac{B}{x} + \dfrac{C}{x^2} + \dfrac{D}{2x - 1}$$
The numerator and denominator both have degree 3. Perform polynomial division first to find $A$, then decompose the remaining proper fraction for $B$, $C$, and $D$.
Expand denominator: $x^2(2x-1) = 2x^3 - x^2$.
Since $\deg(\text{numerator}) = \deg(\text{denominator}) = 3$, perform long division:
So $\dfrac{2x^3 - 1}{2x^3 - x^2} = 1 + \dfrac{x^2 - 1}{2x^3 - x^2}$.
Therefore $A = 1$ and the proper fraction is $\dfrac{x^2 - 1}{x^2(2x-1)}$.
$$\dfrac{x^2 - 1}{x^2(2x-1)} = \dfrac{B}{x} + \dfrac{C}{x^2} + \dfrac{D}{2x-1}$$
Multiply both sides by $x^2(2x-1)$:
$$x^2 - 1 = Bx(2x-1) + C(2x-1) + Dx^2$$
Find $C$: set $x = 0$ (root of $x$) in the identity $x^2 - 1 \equiv Bx(2x-1) + C(2x-1) + Dx^2$:
$$-1 = C(-1) \;\Rightarrow\; C = 1$$
Find $D$: set $x = \dfrac{1}{2}$ (root of $2x-1$):
$$\tfrac{1}{4} - 1 = D \cdot \tfrac{1}{4} \;\Rightarrow\; -\tfrac{3}{4} = \tfrac{D}{4} \;\Rightarrow\; D = -3$$
Substitute $x = 1$ into the identity $x^2-1 = Bx(2x-1) + C(2x-1) + Dx^2$:
$$0 = B(1)(1) + 1(1) + (-3)(1) \;\Rightarrow\; 0 = B + 1 - 3 \;\Rightarrow\; B = 2$$
$$\dfrac{2x^3 - 1}{x^2(2x - 1)} = 1 + \dfrac{2}{x} + \dfrac{1}{x^2} - \dfrac{3}{2x - 1}$$
Exit Ticket
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