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## Homework Statement

Find dy/dx of y=(cosx)^x

## Homework Equations

d/dx of a^u=lna*a^u*u'

## The Attempt at a Solution

I thougt I just had to follow the form shown above, and this is what I got.

y=(cosx)^x

dy/dx=ln(cosx)*(cosx)^x*1

dy/dx=ln(cosx)*(cosx)^x

However, the actual answer is (cosx)^x*(ln(cosx)-xtanx)

I don't understand where this comes from at all. Thanks for your input.