dengan menggunakan rumus cos (alfa + beta) tunjukan bahwa cos (90°- alfa°)=sin alfa°

akan kita tunjukkan bahwa
[tex]\cos{90^{o}-\theta}=\sin{\theta}[/tex]

misalkan
[tex]\alpha =90^{o},\beta =-\theta \\\cos{\left(\alpha +\beta\right) }=\cos{\alpha}\cos{\beta}-\sin{\alpha}\sin{\beta}\\\cos{\left(90^{o}-\theta\right)}=\cos{90^{o}}\cos{\left(-\theta\right)}-\sin{90^{o}}\sin{\left(-\theta\right)}\\=0-1\times (-\sin{\theta})\\=\sin{\theta}[/tex]