cos ( 90 ∘ − α ) = sin ( α ) cos ( 90 ∘ − α ) = a c sin ( α ) = a c ⇓ cos ( 90 ∘ − α ) = sin ( α ) {\displaystyle {\begin{aligned}\cos(90^{\circ }-\alpha )=\sin(\alpha )\\\cos(90^{\circ }-\alpha )={\frac {a}{c}}\\\sin(\alpha )={\frac {a}{c}}\\\Downarrow \\\cos(90^{\circ }-\alpha )=\sin(\alpha )\\\end{aligned}}}
