Maximum and Minimum Value Application question

[shanshan]

Homework Statement

Find the value of k if the function y=x^2+kx+72 has a local minimum at x=4

 

[snipez90]

What have you tried for this problem.

 

[BK201]

Differentiate the original function and you get y = 2x + k ,and it has a local minimun at x=4.
Block 4 in and the output(y) should be 0 because of Fermat's theorem (http://en.wikipedia.org/wiki/Fermat's_theorem_(stationary_points))

So you'll get k = -8 ,is that correct ?

 

[shanshan]

yes! thankyou so much :)