How can i solve this without using my calculator

If a=cos1(1/2)\displaystyle a = cos^{-1}(1/2), then cos(a) = 1/2. That value should be on any chart of values that you should memorize.
 
Do you know a standard acute angle, Θ, such cos(Θ)=1/2 ?
Then the equivalent angle in the second quadrant cos(πθ)=1/2\displaystyle \cos (\pi - \theta ) = - 1/2.
 
Top