Answer
From the figure, we see that x-coordinate in QII is negative of that in QI.
Also, $x = cos\theta$.
Hence, $\cos(180^{\circ}-\theta)=-\cos\theta$.
Work Step by Step
From the figure, we see that x-coordinate in QII is negative of that in QI.
Also, $x = cos\theta$.
Hence, $\cos(180^{\circ}-\theta)=-\cos\theta$.
