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What are the constants a and b so that f=x^3+ax^2+bx+c have a relative maximum at x=-1...

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mccee | Student, Undergraduate | eNoter

Posted February 1, 2012 at 9:04 PM via web

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What are the constants a and b so that f=x^3+ax^2+bx+c have a relative maximum at x=-1 and relative min at x=3?

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sciencesolve | Teacher | (Level 3) Educator Emeritus

Posted February 1, 2012 at 9:42 PM (Answer #1)

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You need to remember that the critical values of a function express the zeroes of derivative of function, hence you need to find derivative of f(x).

`f'(x) = (x^3+ax^2+bx+c)' =gt f'(x) = 3x^2 + 2ax + b`

The problem provides you the following information: the function reaches a relative maximum at x = -1, hence f'(-1)=0

You need to substitute -1 for x in equation of f'(x) such that:

`f'(-1) = 3 - 2a + b =gt 3 - 2a + b = 0 =gt -2a + b = -3`

This equation is not sufficient to find a and b, hence you need one more information. The problem provides this additional information such that the function reaches a relative minimum at x = 3, hence f'(3)=0.

You need to substitute 3 for x in equation of f'(x) such that:

`f'(3) = 27 + 3a + b =gt 3a + b = -27`

Subtracting f'(-1) from f'(3) yields: `6a + b + 2a - b = -27 + 3`

Adding like terms yields: `8a= -24 =gt a = -24/8 =gt a = -3`

You need to substitute -3 for a in equation of f'(-1) such that: 6 + b = -3 => b = -9

Hence, evaluating the values of coefficients a and b under the given conditions yields a = -3; b = -9.

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