− 90 ∘ ⋅ π 180 ∘ = 2 ⋅ 5 ⋅ 3 ⋅ 3 1 ⋅ π 2 ⋅ 2 ⋅ 5 ⋅ 3 ⋅ 3 ⋅ − 1 = − π 2 {\displaystyle -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}}}
