The cubic polynomial f(x)\text{f}(x) is such that the coefficient of x3x^3 is −1-1 and the roots of f(x)=0\text{f}(x) = 0 are 11, kk and k2k^2. It is given that f(x)\text{f}(x) has a remainder of −7-7 when divided by x−2x - 2.

  1. (i)

    Show that k3−2k2−2k−3=0k^3 - 2k^2 - 2k - 3 = 0.

  2. (ii)

    Hence find a value for kk and explain that there are no other real values of kk which satisfy this equation.

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Worked solution

2(i)

1. Formulate the expression for the cubic polynomial $f(x)$ using its roots and leading coefficient.

Since the roots are 11, kk, and k2k^2, and the coefficient of x3x^3 is −1-1, we can write:
f(x)=−1(x−1)(x−k)(x−k2)f(x) = -1(x - 1)(x - k)(x - k^2)

2. Apply the Remainder Theorem to set up an equation.

By the Remainder Theorem, since f(x)f(x) has a remainder of −7-7 when divided by x−2x - 2, we have f(2)=−7f(2) = -7.
−1(2−1)(2−k)(2−k2)=−7-1(2 - 1)(2 - k)(2 - k^2) = -7

3. Expand the equation and simplify to obtain the required result.

−1(1)(4−2k2−2k+k3)=−7-1(1)(4 - 2k^2 - 2k + k^3) = -7
4−2k2−2k+k3=74 - 2k^2 - 2k + k^3 = 7
k3−2k2−2k−3=0(shown)k^3 - 2k^2 - 2k - 3 = 0 \quad \text{(shown)}

Answer

Shown.

2(ii)

1. Find an integer root of the cubic equation using the Factor Theorem

Let P(k)=k3−2k2−2k−3P(k) = k^3 - 2k^2 - 2k - 3.
Try k=3k = 3:
P(3)=(3)3−2(3)2−2(3)−3=27−18−6−3=0P(3) = (3)^3 - 2(3)^2 - 2(3) - 3 = 27 - 18 - 6 - 3 = 0
So k=3k = 3 is a root, and (k−3)(k - 3) is a factor.

2. Factorise the cubic polynomial completely

k3−2k2−2k−3=(k−3)(k2+ak+1)k^3 - 2k^2 - 2k - 3 = (k - 3)(k^2 + ak + 1)
Comparing coefficients of k2k^2:
−3+a=−2  ⟹  a=1-3 + a = -2 \implies a = 1
So, k3−2k2−2k−3=(k−3)(k2+k+1)=0k^3 - 2k^2 - 2k - 3 = (k - 3)(k^2 + k + 1) = 0

3. State the real value of k

From the linear factor, k−3=0  ⟹  k=3k - 3 = 0 \implies k = 3.

4. Show that the quadratic factor has no real roots

For the quadratic factor k2+k+1=0k^2 + k + 1 = 0, we check the discriminant:
Δ=b2−4ac=(1)2−4(1)(1)=1−4=−3\Delta = b^2 - 4ac = (1)^2 - 4(1)(1) = 1 - 4 = -3
Since Δ<0\Delta < 0, the equation k2+k+1=0k^2 + k + 1 = 0 has no real roots. Therefore, k=3k = 3 is the only real value.

Answer

k=3k = 3
There are no other real roots because the discriminant of the remaining quadratic factor is negative.

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