# How can i solve this without using my calculator

##### New member
How can i solve this without a calculator?

Cos ^-1 (1/2)

Cos ^-1 (-1/2)

#### tkhunny

##### Moderator
Staff member
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.

#### pka

##### Elite Member
Do you know a standard acute angle, Θ, such cos(Θ)=1/2 ?
Then the equivalent angle in the second quadrant $$\displaystyle \cos (\pi - \theta ) = - 1/2$$.