Because tan a = sin a/cos a, sin a 2+cos a 2 = 1.

So (COSA * TANA) 2+COSA 2 = 1, TANA = root number 3.

4 cos a 2 =1,cos a 2 =1/4.

cos a=- 1/2

Sina-COSA = COSA * Tana-COSA =-(1/2) * Root number 3+ 1/2=( 1-√3)/2.