The polynomial f(x)\mathrm{f}(x) is such that the coefficient of x4x^4 is 22. The roots of the equation f(x)=0\mathrm{f}(x) = 0 are 32\frac{3}{2} and −1-1. f(x)\mathrm{f}(x) has a remainder of −8-8 when divided by x−1x-1 and a remainder of 2828 when divided by x+2x+2.

  1. (a)

    Find an expression for f(x)\mathrm{f}(x).

  2. (b)

    Show with clear working the number of real roots for f(x)=0\mathrm{f}(x) = 0.

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Worked solution

6(a)

1. Set up the general form of the polynomial using the given roots and leading coefficient.

Since the roots are 32\frac{3}{2} and −1-1, (2x−3)(2x-3) and (x+1)(x+1) are factors of f(x)f(x).
Given that the coefficient of x4x^4 is 22, we can assume f(x)f(x) is a quartic polynomial of the form:
f(x)=(2x−3)(x+1)(x2+px+q)f(x) = (2x-3)(x+1)(x^2 + px + q)

2. Use the Remainder Theorem to form equations for $p$ and $q$.

By the Remainder Theorem, f(1)=−8f(1) = -8 and f(−2)=28f(-2) = 28.
For f(1)=−8f(1) = -8:
(2(1)−3)(1+1)(12+p(1)+q)=−8(2(1)-3)(1+1)(1^2 + p(1) + q) = -8
(−1)(2)(1+p+q)=−8(-1)(2)(1 + p + q) = -8
−2(1+p+q)=−8-2(1 + p + q) = -8
1+p+q=41 + p + q = 4
p+q=3— (1)p + q = 3 \quad \text{--- (1)}

For f(−2)=28f(-2) = 28:
(2(−2)−3)(−2+1)((−2)2+p(−2)+q)=28(2(-2)-3)(-2+1)((-2)^2 + p(-2) + q) = 28
(−7)(−1)(4−2p+q)=28(-7)(-1)(4 - 2p + q) = 28
7(4−2p+q)=287(4 - 2p + q) = 28
4−2p+q=44 - 2p + q = 4
−2p+q=0  ⟹  q=2p— (2)-2p + q = 0 \implies q = 2p \quad \text{--- (2)}

3. Solve the simultaneous equations to find $p$ and $q$.

Substitute (2) into (1):
p+2p=3p + 2p = 3
3p=3  ⟹  p=13p = 3 \implies p = 1

Substitute p=1p = 1 into (2):
q=2(1)=2q = 2(1) = 2

4. Write the final expression for $f(x)$.

Substitute pp and qq back into the expression for f(x)f(x):
f(x)=(2x−3)(x+1)(x2+x+2)f(x) = (2x-3)(x+1)(x^2 + x + 2)
f(x)=(2x2−x−3)(x2+x+2)f(x) = (2x^2 - x - 3)(x^2 + x + 2)
f(x)=2x4+2x3+4x2−x3−x2−2x−3x2−3x−6f(x) = 2x^4 + 2x^3 + 4x^2 - x^3 - x^2 - 2x - 3x^2 - 3x - 6
f(x)=2x4+x3−5x−6f(x) = 2x^4 + x^3 - 5x - 6

Answer

f(x)=2x4+x3−5x−6f(x) = 2x^4 + x^3 - 5x - 6

6(b)

1. Use the expression for $f(x)$ from part (a) to identify the remaining quadratic factor.

From part (a), we found that f(x)=(2x−3)(x+1)(x2+x+2)\mathrm{f}(x) = (2x-3)(x+1)(x^2+x+2).
For f(x)=0\mathrm{f}(x) = 0, the roots are x=32x = \frac{3}{2}, x=−1x = -1, or the roots of x2+x+2=0x^2+x+2 = 0.

2. Evaluate the discriminant of the quadratic factor to check for real roots.

For x2+x+2=0x^2+x+2 = 0, the discriminant is:
b2−4ac=(1)2−4(1)(2)=1−8=−7b^2 - 4ac = (1)^2 - 4(1)(2) = 1 - 8 = -7
Since the discriminant is less than 00, x2+x+2=0x^2+x+2 = 0 has no real roots.

3. State the total number of real roots.

Therefore, the only real roots are 32\frac{3}{2} and −1-1.
There are exactly 2 real roots.

Answer

There are exactly 2 real roots.

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