Show that x−2x - 2 is a factor of x3−2x2+3x−6x^{3} - 2x^{2} + 3x - 6. Express 11−2xx3−2x2+3x−6\dfrac{11 - 2x}{x^{3} - 2x^{2} + 3x - 6} as partial fractions.

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Worked solution

3(i)

1. Evaluate the polynomial at $x = 2$

Let P(x)=x3−2x2+3x−6P(x) = x^3 - 2x^2 + 3x - 6.
P(2)=(2)3−2(2)2+3(2)−6P(2) = (2)^3 - 2(2)^2 + 3(2) - 6
P(2)=8−8+6−6=0P(2) = 8 - 8 + 6 - 6 = 0

2. State the conclusion using the Factor Theorem

Since P(2)=0P(2) = 0, by the Factor Theorem, x−2x - 2 is a factor of x3−2x2+3x−6x^3 - 2x^2 + 3x - 6.

Answer

Shown

3(ii)

1. Factorise the denominator completely using the result from part (i)

From part (i), x−2x - 2 is a factor.
x3−2x2+3x−6=x2(x−2)+3(x−2)=(x−2)(x2+3)x^3 - 2x^2 + 3x - 6 = x^2(x - 2) + 3(x - 2) = (x - 2)(x^2 + 3)

2. Set up the partial fraction decomposition

Let 11−2x(x−2)(x2+3)=Ax−2+Bx+Cx2+3\dfrac{11 - 2x}{(x - 2)(x^2 + 3)} = \dfrac{A}{x - 2} + \dfrac{Bx + C}{x^2 + 3}

3. Form the basic equation by clearing the denominator

11−2x=A(x2+3)+(Bx+C)(x−2)11 - 2x = A(x^2 + 3) + (Bx + C)(x - 2)

4. Find the value of A by substituting $x = 2$

Substitute x=2x = 2:
11−2(2)=A(22+3)+011 - 2(2) = A(2^2 + 3) + 0
7=7A  ⟹  A=17 = 7A \implies A = 1

5. Find the values of B and C by comparing coefficients and substituting values

Compare coefficients of x2x^2:
0=A+B  ⟹  0=1+B  ⟹  B=−10 = A + B \implies 0 = 1 + B \implies B = -1

Substitute x=0x = 0:
11=A(3)+C(−2)11 = A(3) + C(-2)
11=1(3)−2C11 = 1(3) - 2C
2C=−8  ⟹  C=−42C = -8 \implies C = -4

6. Write the final partial fraction expression

11−2xx3−2x2+3x−6=1x−2+−x−4x2+3=1x−2−x+4x2+3\dfrac{11 - 2x}{x^3 - 2x^2 + 3x - 6} = \dfrac{1}{x - 2} + \dfrac{-x - 4}{x^2 + 3} = \dfrac{1}{x - 2} - \dfrac{x + 4}{x^2 + 3}

Answer

1x−2−x+4x2+3\dfrac{1}{x - 2} - \dfrac{x + 4}{x^2 + 3}

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