Originally Posted by

**DavidRUK** Hi

I have a question on a paper that I have to solve though I have become unsure as to what to do.

The problem is as follows

Given that y=16x+x^-1, find the two values of x for which dy/dx = 0.

When I differentiate I get 16 + x^-2 or 16 + 1/x^2 Mr F says: This is wrong. It should be 16 - x^-2 or 16 - 1/x^2.

to remove the fraction I multiply both sides by x^2 giving

16x^2 + 1 = 0

so dy/dx = 0 -> 16x^2 + 1 = 0.

I thought to obtain the two values of x you have to factorise the quadratic, and I am not sure how to do this given the form of the derived equation.

Any help with this would be gratefully appreciated.

Thank You

David