We have y = 8^cos x.

We use the chain rule to find the derivative of 8^(cos x)

Now the derivative of a^x is a^x log a and the derivative of cos x is -sin x.

y' = log 8 * 8^cos x * (cos x)'

=> y' = -log 8 * 8^cos x* sin x

**Therefore dy/dx = -log 8 * 8^cos x* sin x**