A function f is defined by f(x) = x^3 + a x^2 + b x + c , where a, b and c are constants. The graph of y = f(x) passes through the points (-2, 1) and (2, -3). The point (2, 1) lies on the graph of y = f(x + 1). Find the values of a, b and c.

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Worked solution

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1. Form two equations using the points $(-2, 1)$ and $(2, -3)$ on the graph of $y = \mathrm{f}(x)$.

Since the graph of y=f(x)y = \mathrm{f}(x) passes through (−2,1)(-2, 1) and (2,−3)(2, -3), we have f(−2)=1\mathrm{f}(-2) = 1 and f(2)=−3\mathrm{f}(2) = -3.

(−2)3+a(−2)2+b(−2)+c=1(-2)^3 + a(-2)^2 + b(-2) + c = 1
−8+4a−2b+c=1-8 + 4a - 2b + c = 1
4a−2b+c=9— (1)4a - 2b + c = 9 \quad \text{--- (1)}

(2)3+a(2)2+b(2)+c=−3(2)^3 + a(2)^2 + b(2) + c = -3
8+4a+2b+c=−38 + 4a + 2b + c = -3
4a+2b+c=−11— (2)4a + 2b + c = -11 \quad \text{--- (2)}

2. Form a third equation using the point $(2, 1)$ on the graph of $y = \mathrm{f}(x + 1)$.

The point (2,1)(2, 1) lies on y=f(x+1)y = \mathrm{f}(x + 1), so substituting x=2x = 2 and y=1y = 1 yields:
1=f(2+1)  ⟹  f(3)=11 = \mathrm{f}(2 + 1) \implies \mathrm{f}(3) = 1

Substituting x=3x = 3 into f(x)\mathrm{f}(x):
33+a(3)2+b(3)+c=13^3 + a(3)^2 + b(3) + c = 1
27+9a+3b+c=127 + 9a + 3b + c = 1
9a+3b+c=−26— (3)9a + 3b + c = -26 \quad \text{--- (3)}

3. Solve the system of three linear equations to find $a$, $b$, and $c$.

Subtracting (1) from (2):
(4a+2b+c)−(4a−2b+c)=−11−9(4a + 2b + c) - (4a - 2b + c) = -11 - 9
4b=−20  ⟹  b=−54b = -20 \implies b = -5

Substituting b=−5b = -5 into (1) and (3):
4a−2(−5)+c=9  ⟹  4a+c=−1— (4)4a - 2(-5) + c = 9 \implies 4a + c = -1 \quad \text{--- (4)}
9a+3(−5)+c=−26  ⟹  9a+c=−11— (5)9a + 3(-5) + c = -26 \implies 9a + c = -11 \quad \text{--- (5)}

Subtracting (4) from (5):
(9a+c)−(4a+c)=−11−(−1)(9a + c) - (4a + c) = -11 - (-1)
5a=−10  ⟹  a=−25a = -10 \implies a = -2

Substituting a=−2a = -2 into (4):
4(−2)+c=−1  ⟹  −8+c=−1  ⟹  c=74(-2) + c = -1 \implies -8 + c = -1 \implies c = 7

(Note: The system can also be solved directly using a graphing calculator.)

Answer

a=−2a = -2, b=−5b = -5, c=7c = 7

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