Guessing won't help

Algebra Level 3

f ( 1 ) = 1 f ( 1 ) = 11 f ( 2 ) = 22 f ( 2 ) = 14 f\left( 1 \right) =1\\ f\left( -1 \right) =11\\ f\left( -2 \right) =22\\ f\left( 2 \right) =14

If f ( x ) f\left( x \right) is a cubic polynomial satisfying the above equations, find the value of f ( 0 ) f\left( 0 \right)


The answer is 2.

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1 solution

Noel Lo
Jun 10, 2015

Let f ( x ) = a x 3 + b x 2 + c x + d f(x) = ax^3 + bx^2 + cx + d

From the four pieces of information we form the equations:

a + b + c + d = 1 a+b+c+d = 1

a + b c + d = 11 -a+b-c+d = 11

8 a + 4 b 2 c + d = 22 -8a+4b-2c+d = 22

8 a + 4 b + 2 c + d = 14 8a+4b+2c+d = 14

Adding the first to the second and simplifying, we have b + d = 6 b+d = 6 . Doing the same for the other two equations we have 4 b + d = 18 4b+d = 18 . Note that the question is looking for f ( 0 ) f(0) which is simply the value of d d so we need only solve for d d .

( 4 b + d ) ( b + d ) = 18 6 (4b+d) - (b+d) = 18-6

3 b = 12 3b = 12

b = 4 b=4

4 + d = 6 4+d = 6

d = 6 4 = 2 d = 6-4 = \boxed{2} .

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