A polynomial f(x)\mathrm{f}(x) is given by 4x3−6x2+kx+34x^3 - 6x^2 + kx + 3, where kk is a constant.

  1. (a)

    Find the value of kk given that f(x)\mathrm{f}(x) leaves a reminder of 14 when divided by 2x−12x-1.

  2. (b)

    In the case where k=2k = 2, the quadratic expression 2x2+px+32x^2 + px + 3 is a factor of f(x)\mathrm{f}(x). Find the value of the constant pp.

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Worked solution

7(a)

1. Apply the Remainder Theorem by substituting $x = \frac{1}{2}$ into $\mathrm{f}(x)$ and equating to 14.

By the Remainder Theorem, f(12)=14\mathrm{f}\left(\frac{1}{2}\right) = 14.
4(12)3−6(12)2+k(12)+3=144\left(\frac{1}{2}\right)^3 - 6\left(\frac{1}{2}\right)^2 + k\left(\frac{1}{2}\right) + 3 = 14

2. Simplify the equation and solve for $k$.

4(18)−6(14)+k2+3=144\left(\frac{1}{8}\right) - 6\left(\frac{1}{4}\right) + \frac{k}{2} + 3 = 14
12−32+k2+3=14\frac{1}{2} - \frac{3}{2} + \frac{k}{2} + 3 = 14
−1+k2+3=14-1 + \frac{k}{2} + 3 = 14
k2+2=14\frac{k}{2} + 2 = 14
k2=12\frac{k}{2} = 12
k=24k = 24

Answer

k=24k = 24

7(b)

1. Set up the polynomial identity with an unknown linear factor

Since 2x2+px+32x^2 + px + 3 is a quadratic factor of the cubic polynomial f(x)=4x3−6x2+2x+3\mathrm{f}(x) = 4x^3 - 6x^2 + 2x + 3, the other factor must be linear. Let it be ax+bax + b.
4x3−6x2+2x+3=(2x2+px+3)(ax+b)4x^3 - 6x^2 + 2x + 3 = (2x^2 + px + 3)(ax + b)

2. Determine the coefficients of the linear factor

Comparing the coefficients of x3x^3:
4=2a  ⟹  a=24 = 2a \implies a = 2
Comparing the constant terms:
3=3b  ⟹  b=13 = 3b \implies b = 1
So, the identity is:
4x3−6x2+2x+3=(2x2+px+3)(2x+1)4x^3 - 6x^2 + 2x + 3 = (2x^2 + px + 3)(2x + 1)

3. Compare coefficients of $x^2$ (or $x$) to find $p$

Comparing the coefficients of x2x^2:
−6=2(1)+p(2)-6 = 2(1) + p(2)
−6=2+2p-6 = 2 + 2p
2p=−82p = -8
p=−4p = -4

(Check by comparing coefficients of xx: 2=p(1)+3(2)  ⟹  2=p+6  ⟹  p=−42 = p(1) + 3(2) \implies 2 = p + 6 \implies p = -4)

Answer

p=−4p = -4

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