Question

Prove que cos (α- β) = cos α . cosβ + sen α. sen β

Answer 1

Tomemos com exemplos 60° e 30°

Cos(60-30)=cos60*cos30+sen60+sen30

√3/2=1/2*√3/2+√3/2*1/2
√3/2=√3/4+√3/4
√3/2=2√3/4
√3/2 = √3/2