6.1 Angles and Their Measure/45

$-90^{\circ }\cdot {\frac {\pi }{180^{\circ }}}={\frac {{\cancel {2}}\cdot {\cancel {5}}\cdot {\cancel {3}}\cdot {\cancel {3}}}{1}}\cdot {\frac {\pi }{{2}\cdot {\cancel {2}}\cdot {\cancel {5}}\cdot {\cancel {3}}\cdot {\cancel {3}}}}\cdot {-1}=-{\frac {\pi }{2}}$