Functions Again?

Algebra Level 4

Let f ( x ) f(x) be a monic fourth degree polynomial such that f ( 1 ) = 1 f(-1) \ = -1 , f ( 2 ) = 4 f(2) \ = -4 , f ( 3 ) = 9 f(-3) \ = -9 and f ( 4 ) = 16 f(4) \ = -16 , then which of the following is correct?

f ( 0 ) = 24 f(0)=24 f ( 2 ) = 28 f(-2) = -28 f ( 1 ) = 23 f(1) = 23 f ( 0 ) + f ( 2 ) = 4 f(0) + f(-2) = -4 All options are correct

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1 solution

Hung Woei Neoh
Jun 15, 2016

Two methods to solve this:

Method 1 (The lazy but quick method)

Notice that for x = 1 , 2 , 3 , 4 x=-1,2,-3,4

f ( x ) = x 2 f(x) = -x^2

This implies that f ( x ) + x 2 = 0 f(x) +x^2 = 0 has roots x = 1 , 2 , 3 , 4 x=-1,2,-3,4

f ( x ) + x 2 = ( x + 1 ) ( x 2 ) ( x + 3 ) ( x 4 ) f ( x ) = ( x + 1 ) ( x 2 ) ( x + 3 ) ( x 4 ) x 2 \implies f(x) + x^2 = (x+1)(x-2)(x+3)(x-4)\\ f(x) = (x+1)(x-2)(x+3)(x-4) - x^2

Check all the available options:

f ( 1 ) = 23 f ( 0 ) = 24 f ( 2 ) = 28 f ( 0 ) + f ( 2 ) = 24 + ( 28 ) = 4 f(1) = 23\\ f(0) = 24\\ f(-2) = -28\\ f(0) + f(-2) = 24 + (-28) = -4

Therefore, it should be All options are correct \boxed{\text{All options are correct}} (I'm assuming that there is an error in the options)


Method 2 (The more tedious but easier to understand method)

Let f ( x ) = x 4 + a x 3 + b x 2 + c x + d f(x) = x^4 +ax^3 + bx^2+cx+d (Monic means that the leading coefficient is 1)

Substitute the values of x x and f ( x ) f(x) into the equation above, and you will get:

a + b c + d = 2 8 a + 4 b + 2 c + d = 20 27 a + 9 b 3 c + d = 90 64 a + 16 b + 4 c + d = 272 -a+b-c+d=-2\\ 8a+4b+2c+d=-20\\ -27a+9b-3c+d = -90\\ 64a+16b+4c+d = -272

Solve this system of linear equations, and you should get a = 2 , b = 14 , c = 14 , d = 24 a=-2,b=-14,c=14,d=24

Therefore, f ( x ) = x 4 2 x 3 14 x 2 + 14 x + 24 f(x) = x^4-2x^3-14x^2+14x+24

Check all the available options:

f ( 1 ) = 23 f ( 0 ) = 24 f ( 2 ) = 28 f ( 0 ) + f ( 2 ) = 24 + ( 28 ) = 4 f(1) = 23\\ f(0) = 24\\ f(-2) = -28\\ f(0) + f(-2) = 24 + (-28) = -4

Therefore, it should be All options are correct \boxed{\text{All options are correct}} (I'm assuming that there is an error in the options)

f ( 1 ) = 23 24 f(1)=23\neq-24 , so not all options are correct.

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I said I'm assuming that there is an error in the options

Hung Woei Neoh - 5 years ago

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Your solution is correct. My comment was just to point out to @Rishabh Tiwari that there was an error.

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