Given that f(x)=2x3+px2+7x+qf(x) = 2x^3 + px^2 + 7x + q. Find the value of pp and of qq for which (x−3)(x-3) and (x−2)(x-2) are factors of f(x)f(x). Hence solve f(x)=0f(x) = 0.

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Worked solution

1

1. Apply the Factor Theorem for $(x-3)$ to form the first equation.

Since (x−3)(x-3) is a factor, f(3)=0f(3) = 0.
2(3)3+p(3)2+7(3)+q=02(3)^3 + p(3)^2 + 7(3) + q = 0
54+9p+21+q=054 + 9p + 21 + q = 0
9p+q=−75— (1)9p + q = -75 \quad \text{--- (1)}

2. Apply the Factor Theorem for $(x-2)$ to form the second equation.

Since (x−2)(x-2) is a factor, f(2)=0f(2) = 0.
2(2)3+p(2)2+7(2)+q=02(2)^3 + p(2)^2 + 7(2) + q = 0
16+4p+14+q=016 + 4p + 14 + q = 0
4p+q=−30— (2)4p + q = -30 \quad \text{--- (2)}

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

Subtracting (2) from (1):
(9p+q)−(4p+q)=−75−(−30)(9p + q) - (4p + q) = -75 - (-30)
5p=−455p = -45
p=−9p = -9

Substitute p=−9p = -9 into (2):
4(−9)+q=−304(-9) + q = -30
−36+q=−30-36 + q = -30
q=6q = 6

4. Find the remaining linear factor of $f(x)$.

The polynomial is f(x)=2x3−9x2+7x+6f(x) = 2x^3 - 9x^2 + 7x + 6.
Since (x−3)(x-3) and (x−2)(x-2) are factors, their product (x−3)(x−2)=x2−5x+6(x-3)(x-2) = x^2 - 5x + 6 is a quadratic factor of f(x)f(x).
Let the third linear factor be (ax+b)(ax + b).
2x3−9x2+7x+6=(x2−5x+6)(ax+b)2x^3 - 9x^2 + 7x + 6 = (x^2 - 5x + 6)(ax + b)
Comparing the coefficients of x3x^3: a=2a = 2.
Comparing the constant terms: 6b=6  ⟹  b=16b = 6 \implies b = 1.
So, the third factor is (2x+1)(2x + 1).

5. Solve $f(x) = 0$.

f(x)=(x−3)(x−2)(2x+1)=0f(x) = (x-3)(x-2)(2x+1) = 0
x−3=0  ⟹  x=3x - 3 = 0 \implies x = 3
x−2=0  ⟹  x=2x - 2 = 0 \implies x = 2
2x+1=0  ⟹  x=−122x + 1 = 0 \implies x = -\frac{1}{2}

Answer

p=−9p = -9, q=6q = 6
x=3x = 3, x=2x = 2, or x=−12x = -\frac{1}{2}

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