Precalculus Examples

$\frac{x^{2} - 11 x - 18}{x (x^{2} + 3 x + 3)}$

Step 1

Decompose the fraction and multiply through by the common denominator.

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Step 1.1

For each factor in the denominator, create a new fraction using the factor as the denominator, and an unknown value as the numerator. Since the factor is 2nd order, $2$ terms are required in the numerator. The number of terms required in the numerator is always equal to the order of the factor in the denominator.

$\frac{A}{x} + \frac{B x + C}{x^{2} + 3 x + 3}$

Step 1.2

Multiply each fraction in the equation by the denominator of the original expression. In this case, the denominator is $x (x^{2} + 3 x + 3)$ .

$\frac{(x^{2} - 11 x - 18) (x (x^{2} + 3 x + 3))}{x (x^{2} + 3 x + 3)} = \frac{A (x (x^{2} + 3 x + 3))}{x} + \frac{(B x + C) (x (x^{2} + 3 x + 3))}{x^{2} + 3 x + 3}$

Step 1.3

Cancel the common factor of $x$ .

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Step 1.3.1

Cancel the common factor.

$\frac{(x^{2} - 11 x - 18) (x (x^{2} + 3 x + 3))}{x (x^{2} + 3 x + 3)} = \frac{A (x (x^{2} + 3 x + 3))}{x} + \frac{(B x + C) (x (x^{2} + 3 x + 3))}{x^{2} + 3 x + 3}$

Step 1.3.2

Rewrite the expression.

$\frac{(x^{2} - 11 x - 18) (x^{2} + 3 x + 3)}{x^{2} + 3 x + 3} = \frac{A (x (x^{2} + 3 x + 3))}{x} + \frac{(B x + C) (x (x^{2} + 3 x + 3))}{x^{2} + 3 x + 3}$

Step 1.4

Cancel the common factor of $x^{2} + 3 x + 3$ .

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Step 1.4.1

Cancel the common factor.

$\frac{(x^{2} - 11 x - 18) (x^{2} + 3 x + 3)}{x^{2} + 3 x + 3} = \frac{A (x (x^{2} + 3 x + 3))}{x} + \frac{(B x + C) (x (x^{2} + 3 x + 3))}{x^{2} + 3 x + 3}$

Step 1.4.2

Divide $x^{2} - 11 x - 18$ by $1$ .

$x^{2} - 11 x - 18 = \frac{A (x (x^{2} + 3 x + 3))}{x} + \frac{(B x + C) (x (x^{2} + 3 x + 3))}{x^{2} + 3 x + 3}$

Step 1.5

Simplify each term.

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Step 1.5.1

Cancel the common factor of $x$ .

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Step 1.5.1.1

Cancel the common factor.

$x^{2} - 11 x - 18 = \frac{A (x (x^{2} + 3 x + 3))}{x} + \frac{(B x + C) (x (x^{2} + 3 x + 3))}{x^{2} + 3 x + 3}$

Step 1.5.1.2

Divide $A (x^{2} + 3 x + 3)$ by $1$ .

$x^{2} - 11 x - 18 = A (x^{2} + 3 x + 3) + \frac{(B x + C) (x (x^{2} + 3 x + 3))}{x^{2} + 3 x + 3}$

Step 1.5.2

Apply the distributive property.

$x^{2} - 11 x - 18 = A x^{2} + A (3 x) + A \cdot 3 + \frac{(B x + C) (x (x^{2} + 3 x + 3))}{x^{2} + 3 x + 3}$

Step 1.5.3

Simplify.

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Step 1.5.3.1

Rewrite using the commutative property of multiplication.

$x^{2} - 11 x - 18 = A x^{2} + 3 A x + A \cdot 3 + \frac{(B x + C) (x (x^{2} + 3 x + 3))}{x^{2} + 3 x + 3}$

Step 1.5.3.2

Move $3$ to the left of $A$ .

$x^{2} - 11 x - 18 = A x^{2} + 3 A x + 3 \cdot A + \frac{(B x + C) (x (x^{2} + 3 x + 3))}{x^{2} + 3 x + 3}$

Step 1.5.4

Cancel the common factor of $x^{2} + 3 x + 3$ .

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Step 1.5.4.1

Cancel the common factor.

$x^{2} - 11 x - 18 = A x^{2} + 3 A x + 3 A + \frac{(B x + C) (x (x^{2} + 3 x + 3))}{x^{2} + 3 x + 3}$

Step 1.5.4.2

Divide $(B x + C) (x)$ by $1$ .

$x^{2} - 11 x - 18 = A x^{2} + 3 A x + 3 A + (B x + C) (x)$

Step 1.5.5

Apply the distributive property.

$x^{2} - 11 x - 18 = A x^{2} + 3 A x + 3 A + B x \cdot x + C x$

Step 1.5.6

Multiply $x$ by $x$ by adding the exponents.

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Step 1.5.6.1

Move $x$ .

$x^{2} - 11 x - 18 = A x^{2} + 3 A x + 3 A + B (x \cdot x) + C x$

Step 1.5.6.2

Multiply $x$ by $x$ .

$x^{2} - 11 x - 18 = A x^{2} + 3 A x + 3 A + B x^{2} + C x$

Step 1.6

Simplify the expression.

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Step 1.6.1

Move $A$ .

$x^{2} - 11 x - 18 = A x^{2} + 3 x A + 3 A + B x^{2} + C x$

Step 1.6.2

Reorder $B$ and $x^{2}$ .

$x^{2} - 11 x - 18 = A x^{2} + 3 x A + 3 A + B x^{2} + C x$

Step 1.6.3

Move $3 A$ .

$x^{2} - 11 x - 18 = A x^{2} + 3 x A + B x^{2} + C x + 3 A$

Step 1.6.4

Move $3 x A$ .

$x^{2} - 11 x - 18 = A x^{2} + B x^{2} + 3 x A + C x + 3 A$

Step 2

Create equations for the partial fraction variables and use them to set up a system of equations.

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Step 2.1

Create an equation for the partial fraction variables by equating the coefficients of $x^{2}$ from each side of the equation. For the equation to be equal, the equivalent coefficients on each side of the equation must be equal.

$1 = A + B$

Step 2.2

Create an equation for the partial fraction variables by equating the coefficients of $x$ from each side of the equation. For the equation to be equal, the equivalent coefficients on each side of the equation must be equal.

$- 11 = 3 A + C$

Step 2.3

Create an equation for the partial fraction variables by equating the coefficients of the terms not containing $x$ . For the equation to be equal, the equivalent coefficients on each side of the equation must be equal.

$- 18 = 3 A$

Step 2.4

Set up the system of equations to find the coefficients of the partial fractions.

$1 = A + B$

$- 11 = 3 A + C$

$- 18 = 3 A$

$1 = A + B$

$- 11 = 3 A + C$

$- 18 = 3 A$

Step 3

Solve the system of equations.

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Step 3.1

Solve for $A$ in $- 18 = 3 A$ .

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Step 3.1.1

Rewrite the equation as $3 A = - 18$ .

$3 A = - 18$

$1 = A + B$

$- 11 = 3 A + C$

Step 3.1.2

Divide each term in $3 A = - 18$ by $3$ and simplify.

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Step 3.1.2.1

Divide each term in $3 A = - 18$ by $3$ .

$\frac{3 A}{3} = \frac{- 18}{3}$

$1 = A + B$

$- 11 = 3 A + C$

Step 3.1.2.2

Simplify the left side.

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Step 3.1.2.2.1

Cancel the common factor of $3$ .

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Step 3.1.2.2.1.1

Cancel the common factor.

$\frac{3 A}{3} = \frac{- 18}{3}$

$1 = A + B$

$- 11 = 3 A + C$

Step 3.1.2.2.1.2

Divide $A$ by $1$ .

$A = \frac{- 18}{3}$

$1 = A + B$

$- 11 = 3 A + C$

$A = \frac{- 18}{3}$

$1 = A + B$

$- 11 = 3 A + C$

$A = \frac{- 18}{3}$

$1 = A + B$

$- 11 = 3 A + C$

Step 3.1.2.3

Simplify the right side.

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Step 3.1.2.3.1

Divide $- 18$ by $3$ .

$A = - 6$

$1 = A + B$

$- 11 = 3 A + C$

$A = - 6$

$1 = A + B$

$- 11 = 3 A + C$

$A = - 6$

$1 = A + B$

$- 11 = 3 A + C$

$A = - 6$

$1 = A + B$

$- 11 = 3 A + C$

Step 3.2

Replace all occurrences of $A$ with $- 6$ in each equation.

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Step 3.2.1

Replace all occurrences of $A$ in $1 = A + B$ with $- 6$ .

$1 = (- 6) + B$

$A = - 6$

$- 11 = 3 A + C$

Step 3.2.2

Simplify the right side.

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Step 3.2.2.1

Remove parentheses.

$1 = - 6 + B$

$A = - 6$

$- 11 = 3 A + C$

$1 = - 6 + B$

$A = - 6$

$- 11 = 3 A + C$

Step 3.2.3

Replace all occurrences of $A$ in $- 11 = 3 A + C$ with $- 6$ .

$- 11 = 3 (- 6) + C$

$1 = - 6 + B$

$A = - 6$

Step 3.2.4

Simplify the right side.

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Step 3.2.4.1

Multiply $3$ by $- 6$ .

$- 11 = - 18 + C$

$1 = - 6 + B$

$A = - 6$

$- 11 = - 18 + C$

$1 = - 6 + B$

$A = - 6$

$- 11 = - 18 + C$

$1 = - 6 + B$

$A = - 6$

Step 3.3

Solve for $C$ in $- 11 = - 18 + C$ .

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Step 3.3.1

Rewrite the equation as $- 18 + C = - 11$ .

$- 18 + C = - 11$

$1 = - 6 + B$

$A = - 6$

Step 3.3.2

Move all terms not containing $C$ to the right side of the equation.

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Step 3.3.2.1

Add $18$ to both sides of the equation.

$C = - 11 + 18$

$1 = - 6 + B$

$A = - 6$

Step 3.3.2.2

Add $- 11$ and $18$ .

$C = 7$

$1 = - 6 + B$

$A = - 6$

$C = 7$

$1 = - 6 + B$

$A = - 6$

$C = 7$

$1 = - 6 + B$

$A = - 6$

Step 3.4

Replace all occurrences of $C$ with $7$ in each equation.

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Step 3.4.1

Rewrite the equation as $- 6 + B = 1$ .

$- 6 + B = 1$

$C = 7$

$A = - 6$

Step 3.4.2

Move all terms not containing $B$ to the right side of the equation.

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Step 3.4.2.1

Add $6$ to both sides of the equation.

$B = 1 + 6$

$C = 7$

$A = - 6$

Step 3.4.2.2

Add $1$ and $6$ .

$B = 7$

$C = 7$

$A = - 6$

$B = 7$

$C = 7$

$A = - 6$

$B = 7$

$C = 7$

$A = - 6$

Step 3.5

List all of the solutions.

$B = 7, C = 7, A = - 6$

Step 4

Replace each of the partial fraction coefficients in $\frac{A}{x} + \frac{B x + C}{x^{2} + 3 x + 3}$ with the values found for $A$ , $B$ , and $C$ .

$- \frac{6}{x} + \frac{7 x + 7}{x^{2} + 3 x + 3}$

Step 5

Multiply $7$ by $x$ .

$\frac{- 6}{x} + \frac{7 x + 7}{x^{2} + 3 x + 3}$