Find the absolute maximum and absolute minimum values of f on the given interval.

Find the absolute maximum and absolute minimum values of f on the given interval. (Round all answers to two decimal places.)

f(t) = 2 cos t + sin 2t

[0, ( pi)/2]

so i know the deriv is -2(sint)+cos(2t), but i don't understand how to find a value which makes this zero

Use calculus to find the absolute maximum and minimum values of the function. (Round all answers to three decimal places.)

f(x) = x + 2cos(x)

[0, 2]

again, I know the derivative is 1+-2sinx, but i don't understand how to get a value for x which makes the equation equal to zero.