$\begin{array}{1 1}(a)\;\cos(180^{\large\circ}-\theta)=\large\frac{r_1^2+r_2^2-d^2}{2r_1r_2}\\(b)\;\cos(180^{\large\circ}+\theta)=\large\frac{r_1^2-r_2^2-d^2}{2r_1r_2}\\(c)\;\cos(180^{\large\circ}-\theta)=\large\frac{r_1^2-r_2^2+d^2}{r_1r_2}\\(d)\;\cos(180^{\large\circ}-\theta)=\large\frac{r_1^2+r_2^2+d^2}{4r_1r_2}\end{array}$
