Express 2x4+x3+9x2+7x+3(x+1)(x2+5)\frac{2x^4 + x^3 + 9x^2 + 7x + 3}{(x+1)(x^2+5)} in partial fractions.

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Worked solution

6

1. Expand the denominator and perform long division to extract the polynomial part, as the fraction is improper.

Denominator: (x+1)(x2+5)=x3+x2+5x+5(x+1)(x^2+5) = x^3 + x^2 + 5x + 5

By long division of (2x4+x3+9x2+7x+3)(2x^4 + x^3 + 9x^2 + 7x + 3) by (x3+x2+5x+5)(x^3 + x^2 + 5x + 5):
2x(x3+x2+5x+5)=2x4+2x3+10x2+10x2x(x^3 + x^2 + 5x + 5) = 2x^4 + 2x^3 + 10x^2 + 10x
Subtracting this from the numerator gives:
−x3−x2−3x+3-x^3 - x^2 - 3x + 3
−1(x3+x2+5x+5)=−x3−x2−5x−5-1(x^3 + x^2 + 5x + 5) = -x^3 - x^2 - 5x - 5
Subtracting this gives the remainder:
2x+82x + 8

So, 2x4+x3+9x2+7x+3(x+1)(x2+5)=2x−1+2x+8(x+1)(x2+5)\frac{2x^4 + x^3 + 9x^2 + 7x + 3}{(x+1)(x^2+5)} = 2x - 1 + \frac{2x + 8}{(x+1)(x^2+5)}

2. Set up the partial fraction decomposition for the proper fraction remainder.

Let 2x+8(x+1)(x2+5)=Ax+1+Bx+Cx2+5\frac{2x + 8}{(x+1)(x^2+5)} = \frac{A}{x+1} + \frac{Bx+C}{x^2+5}

Multiplying by (x+1)(x2+5)(x+1)(x^2+5):
2x+8=A(x2+5)+(Bx+C)(x+1)2x + 8 = A(x^2+5) + (Bx+C)(x+1)

3. Solve for the constants $A$, $B$, and $C$.

Substitute x=−1x = -1:
2(−1)+8=A((−1)2+5)+02(-1) + 8 = A((-1)^2+5) + 0
6=6A  ⟹  A=16 = 6A \implies A = 1

Substitute x=0x = 0:
8=1(5)+C(1)8 = 1(5) + C(1)
8=5+C  ⟹  C=38 = 5 + C \implies C = 3

Substitute x=1x = 1:
2(1)+8=1(1+5)+(B(1)+3)(1+1)2(1) + 8 = 1(1+5) + (B(1)+3)(1+1)
10=6+2(B+3)10 = 6 + 2(B+3)
4=2B+6  ⟹  2B=−2  ⟹  B=−14 = 2B + 6 \implies 2B = -2 \implies B = -1

4. Write the final expression in partial fractions.

2x4+x3+9x2+7x+3(x+1)(x2+5)=2x−1+1x+1+3−xx2+5\frac{2x^4 + x^3 + 9x^2 + 7x + 3}{(x+1)(x^2+5)} = 2x - 1 + \frac{1}{x+1} + \frac{3-x}{x^2+5}

Answer

2x−1+1x+1+3−xx2+52x - 1 + \frac{1}{x+1} + \frac{3-x}{x^2+5}

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