Draw the base of a triangle from the origin … to

Draw the base of a triangle from the origin $O=(0,0)$ to $P=(a,0)$. The third corner is at $Q=(b\cos \theta ,b\sin \theta )$. What are the side lengths $OP$ and $OQ$? From the distance formula (1) show that the side $PQ$ has length
$$d^2=a^2+b^2-2ab\cos \theta \text{(law of cosines)}.$$

Gilbert Strang, Calculus, 3rd edition (Wellesley-Cambridge Press; MIT OpenCourseWare RES.18-001) · §1.5 A Review of Trigonometry, #31