You can easily memorise the special angles in a grid (have a look on google) or you can work them out manually buy the use of two triangles.

The first is an equilateral triangle of sides 2cm.

Split it in half to make right angled triangles:

Sides - 2cm 1cm root3cm

Angles - 30 60 90

The second is an Isosceles right-angled triangle with

sides - 1cm 1cm root2cm

Angles - 90 45 45

Then use trig... more