cos 2 θ + sin 2 θ = 1 {\displaystyle \cos ^{2}\theta +\sin ^{2}\theta =1}
cos 2 θ − sin 2 θ = cos 2 θ {\displaystyle \cos ^{2}\theta -\sin ^{2}\theta =\cos 2\theta }
a sin x + b cos x = a 2 + b 2 sin ( x + sin − 1 b a 2 + b 2 ) {\displaystyle a\sin x+b\cos x={\sqrt {a^{2}+b^{2}}}\sin(x+\sin ^{-1}{\frac {b}{\sqrt {a^{2}+b^{2}}}})}
Two equation can be expanded by using the commutative law, associative law and distributive law.
