Well the method I propose uses routine algebra and recognition of special angles and values.

Using the null factor law (you might know it by a different name):

.... (1)

or

.... (2)

Regarding equation (1), an obvious exact solution is x = 0. The tricky bit is to show that this actually theonlysolution ......

Edit: Deleted my remarks on equation (2) for reasons made obvious by an ancient Greek philosopher!