Question 181070
things you should know:
cos30= sqr(3)/2
cos60= 1/2

2cos^2(30) - 1 = cos60
2(cos30)^2 -1 
2(sqr(3)/2)^2 -1
2(3/4) -1 
3/2 -1
1/2
1/2 = cos60