Question

найдите cos2α,если cos α= 0.4

Answer 1

cos 2α = 1-2sin^2 α

sin^2 α=1-cos^2 α

cos^2 α=0.4*0.4=0.16

sin^2 α=1-0.16=0.84

cos^2 α = 1-2*0.84=-0.68