Radian Measure and ANgles of the Cartesion question problem

For the following value of $\displaystyle cos \theta$, determine the radian value of $\displaystyle \theta$

a) -1/2

First I found y from the equation $\displaystyle r^2=x^2+y^2$
y= +- \sqrt{y}

My graph therefore is in the 1 and 2 quadrant due to y having + and - values

then i found inverse cos^-1 (1/2) = 1.05

so: pi - 1.05 or 1.05

However the answer according to the book is (4pi)/3

Scratch this, I managed to figure it out. I forgot that cos is the x value and sin is the y value on the Cartesian Plane