I have this homework question that asks.

Find the values of a, b, c and d, such that g(x) = ax^3 +bx^2 +cx +d has a local max at (2,4) and a local min at (0,0)

Its really confusing me in how to approach it.

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- Dec 21st 2006, 03:37 PMSky12Max and min values help.
I have this homework question that asks.

Find the values of a, b, c and d, such that g(x) = ax^3 +bx^2 +cx +d has a local max at (2,4) and a local min at (0,0)

Its really confusing me in how to approach it. - Dec 21st 2006, 03:57 PMgalactus
Use the equation and its derivative to build a system of equations. It should be rather simplistic.

The max and min occur at (0,0) and (2,4)

We have:

$\displaystyle a(0)^{3}+b(0)^{2}+c(0)+d=0$, therefore, d=0.

$\displaystyle a(2)^{3}+b(2)^{2}+c(2)+d=4$

Derivatives must be 0 at extrema:

$\displaystyle 3a(2)^{2}+2b(2)+c=0$

$\displaystyle 3a(0)^{2}+2b(0)+c=0$, therefore, c=0.

Because c and d are 0, that helps whittle things down a good bit.

We have the following system to solve:

$\displaystyle 8a+4b=4$

$\displaystyle 12a+4b=0$

We find a=-1 and b=3

Therefore, the polynomial is $\displaystyle g(x)=-x^{3}+3x^{2}$