You can use the identity to get and . (Use the positive root in this case since the angles are all in the first quadrant.) Once you have these you can use the properties cos(pi-x) = -cos x and sin(pi-x) = sin x.
For the second problem just plug in the given x value and use special right triangles or special unit circle values to evaluate it. 3 sin (2pi/12) = 3 sin (pi/6) = 3 * 1/2