The polynomial f(x)=Ax3−(2A+B)x2+2x+B\mathrm{f}(x) = Ax^3 - (2A + B)x^2 + 2x + B, where AA and BB are constants, is exactly divisible by (x−1)(x - 1).
When f(x)\mathrm{f}(x) is divided by xx, the remainder obtained is 3.

  1. (a)

    Find AA and BB.

  2. (b)

    Factorise f(x)\mathrm{f}(x) completely.

  3. (c)

    Hence, or otherwise factorise
    g(x)=A(x−2)3−(2A+B)(x−2)2+2(x−2)+B.g(x) = A(x-2)^3 - (2A+B)(x-2)^2 + 2(x-2) + B.

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Worked solution

6(a)

1. Use the Remainder Theorem to find $B$.

Since the remainder is 3 when f(x)\mathrm{f}(x) is divided by xx, we have f(0)=3\mathrm{f}(0) = 3.
f(0)=A(0)3−(2A+B)(0)2+2(0)+B=3\mathrm{f}(0) = A(0)^3 - (2A + B)(0)^2 + 2(0) + B = 3
B=3B = 3

2. Use the Factor Theorem to set up an equation for $A$.

Since f(x)\mathrm{f}(x) is exactly divisible by (x−1)(x - 1), we have f(1)=0\mathrm{f}(1) = 0.
f(1)=A(1)3−(2A+B)(1)2+2(1)+B=0\mathrm{f}(1) = A(1)^3 - (2A + B)(1)^2 + 2(1) + B = 0
A−(2A+B)+2+B=0A - (2A + B) + 2 + B = 0

3. Solve for $A$.

A−2A−B+2+B=0A - 2A - B + 2 + B = 0
−A+2=0-A + 2 = 0
A=2A = 2

Answer

A=2A = 2, B=3B = 3

6(b)

1. Determine the values of $A$ and $B$ from the conditions in part (a) to write the full polynomial.

From part (a), f(0)=3  ⟹  B=3\mathrm{f}(0) = 3 \implies B = 3.
Since (x−1)(x-1) is a factor, f(1)=0  ⟹  A(1)3−(2A+3)(1)2+2(1)+3=0  ⟹  −A+2=0  ⟹  A=2\mathrm{f}(1) = 0 \implies A(1)^3 - (2A + 3)(1)^2 + 2(1) + 3 = 0 \implies -A + 2 = 0 \implies A = 2.
Thus, f(x)=2x3−7x2+2x+3\mathrm{f}(x) = 2x^3 - 7x^2 + 2x + 3.

2. Find the quadratic factor by dividing $\mathrm{f}(x)$ by $(x - 1)$.

Let 2x3−7x2+2x+3=(x−1)(2x2+cx−3)2x^3 - 7x^2 + 2x + 3 = (x - 1)(2x^2 + cx - 3).
Comparing the coefficients of x2x^2:
−2+c=−7  ⟹  c=−5-2 + c = -7 \implies c = -5.
So, the quadratic factor is 2x2−5x−32x^2 - 5x - 3.

3. Factorise the quadratic factor completely.

2x2−5x−3=(2x+1)(x−3)2x^2 - 5x - 3 = (2x + 1)(x - 3)

4. State the completely factorised form of $\mathrm{f}(x)$.

f(x)=(x−1)(2x+1)(x−3)\mathrm{f}(x) = (x - 1)(2x + 1)(x - 3)

Answer

(x−1)(2x+1)(x−3)(x - 1)(2x + 1)(x - 3)

6(c)

1. Recall the factorised form of $f(x)$ from part (b)

From parts (a) and (b), A=2A=2, B=3B=3, and f(x)=2x3−7x2+2x+3=(x−1)(2x+1)(x−3)\mathrm{f}(x) = 2x^3 - 7x^2 + 2x + 3 = (x-1)(2x+1)(x-3).

2. Recognise that $g(x)$ is $f(x-2)$ and substitute $x-2$ into the factorised form

Notice that g(x)=f(x−2)g(x) = \mathrm{f}(x-2).
Substituting (x−2)(x-2) for xx in the factorised form of f(x)\mathrm{f}(x):
g(x)=((x−2)−1)(2(x−2)+1)((x−2)−3)g(x) = ((x-2) - 1)(2(x-2) + 1)((x-2) - 3)

3. Simplify the factors to obtain the final answer

g(x)=(x−3)(2x−4+1)(x−5)g(x) = (x - 3)(2x - 4 + 1)(x - 5)
g(x)=(x−3)(2x−3)(x−5)g(x) = (x - 3)(2x - 3)(x - 5)

Answer

(x−3)(2x−3)(x−5)(x-3)(2x-3)(x-5)

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