thanks
You can think of this as a right triangle with side opposite $\displaystyle \alpha$ 1 and hypotenuse 4. What is the length of the side near $\displaystyle \alpha$, the third side of this right triangle?
You could also use the fact that $\displaystyle sin^2(\theta)+ cos^2(\theta)= 1$ so that $\displaystyle cos(\theta)= \sqrt{1- sin^2(\theta)}$