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