How can i solve this without using my calculator

If \(\displaystyle a = cos^{-1}(1/2)\), 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 \(\displaystyle \cos (\pi - \theta ) = - 1/2\).
 
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