By using long division, show that 2x2+12x^2+1 is a factor of 2x3−4x2+x−22x^3-4x^2+x-2. Express −5x2+11x−112x3−4x2+x−2\dfrac{-5x^2+11x-11}{2x^3-4x^2+x-2} in partial fractions.

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Worked solution

6(a)

1. Perform polynomial long division of $2x^3-4x^2+x-2$ by $2x^2+1$.

\multicolumn1rx−2\cline2−22x2+12x3−4x2+x−22x3+x\cline2−2−4x2−2−4x2−2\cline2−20\begin{array}{r|l} \multicolumn{1}{r}{x - 2} \\ \cline{2-2} 2x^2 + 1 & 2x^3 - 4x^2 + x - 2 \\ & 2x^3 \quad\quad\quad + x \\ \cline{2-2} & \quad\quad -4x^2 \quad\quad - 2 \\ & \quad\quad -4x^2 \quad\quad - 2 \\ \cline{2-2} & \quad\quad\quad\quad\quad\quad\quad 0 \end{array}

2. Conclude that the divisor is a factor based on the zero remainder.

Since the remainder is 00, 2x2+12x^2+1 is a factor of 2x3−4x2+x−22x^3-4x^2+x-2.

Answer

Since the remainder is 00, 2x2+12x^2+1 is a factor of 2x3−4x2+x−22x^3-4x^2+x-2.

6(b)

1. Factorise the denominator using the result from part (a).

From part (a), 2x2+12x^2+1 is a factor of 2x3−4x2+x−22x^3-4x^2+x-2.
By inspection or division, 2x3−4x2+x−2=(2x2+1)(x−2)2x^3-4x^2+x-2 = (2x^2+1)(x-2).

2. Set up the partial fraction decomposition.

Let −5x2+11x−11(2x2+1)(x−2)=Ax+B2x2+1+Cx−2\dfrac{-5x^2+11x-11}{(2x^2+1)(x-2)} = \dfrac{Ax+B}{2x^2+1} + \dfrac{C}{x-2}.

3. Form the basic equation.

Multiplying throughout by (2x2+1)(x−2)(2x^2+1)(x-2):
−5x2+11x−11=(Ax+B)(x−2)+C(2x2+1)-5x^2+11x-11 = (Ax+B)(x-2) + C(2x^2+1)

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

Substitute x=2x = 2:
−5(2)2+11(2)−11=C(2(2)2+1)-5(2)^2 + 11(2) - 11 = C(2(2)^2 + 1)
−20+22−11=9C-20 + 22 - 11 = 9C
−9=9C  ⟹  C=−1-9 = 9C \implies C = -1

Substitute x=0x = 0:
−11=B(−2)+C(1)-11 = B(-2) + C(1)
−11=−2B−1  ⟹  −2B=−10  ⟹  B=5-11 = -2B - 1 \implies -2B = -10 \implies B = 5

Compare coefficients of x2x^2:
−5=A+2C-5 = A + 2C
−5=A+2(−1)  ⟹  A=−3-5 = A + 2(-1) \implies A = -3

5. Write the final partial fraction expression.

−5x2+11x−112x3−4x2+x−2=−3x+52x2+1−1x−2\dfrac{-5x^2+11x-11}{2x^3-4x^2+x-2} = \dfrac{-3x+5}{2x^2+1} - \dfrac{1}{x-2}

Answer

−3x+52x2+1−1x−2\dfrac{-3x+5}{2x^2+1} - \dfrac{1}{x-2}

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