Let be a monic fourth degree polynomial such that , , and , then which of the following is correct?
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Two methods to solve this:
Method 1 (The lazy but quick method)
Notice that for x = − 1 , 2 , − 3 , 4
f ( x ) = − x 2
This implies that f ( x ) + x 2 = 0 has roots 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
Check all the available options:
f ( 1 ) = 2 3 f ( 0 ) = 2 4 f ( − 2 ) = − 2 8 f ( 0 ) + f ( − 2 ) = 2 4 + ( − 2 8 ) = − 4
Therefore, it should be 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 (Monic means that the leading coefficient is 1)
Substitute the values of x and f ( x ) into the equation above, and you will get:
− a + b − c + d = − 2 8 a + 4 b + 2 c + d = − 2 0 − 2 7 a + 9 b − 3 c + d = − 9 0 6 4 a + 1 6 b + 4 c + d = − 2 7 2
Solve this system of linear equations, and you should get a = − 2 , b = − 1 4 , c = 1 4 , d = 2 4
Therefore, f ( x ) = x 4 − 2 x 3 − 1 4 x 2 + 1 4 x + 2 4
Check all the available options:
f ( 1 ) = 2 3 f ( 0 ) = 2 4 f ( − 2 ) = − 2 8 f ( 0 ) + f ( − 2 ) = 2 4 + ( − 2 8 ) = − 4
Therefore, it should be All options are correct (I'm assuming that there is an error in the options)