Question

已知sinα＝5分之根号5,则cos4α的值是多少 help

Answer 1

sina=√5/5

所以(sina)^2=(√5/5)=1/5

所以cosa=±√(1-(sina)^2)=±2√5/5

cos4α=1-2(sin2α)^2

sin2α=2sinacosa=±4/5

所以

cos4α=1-2(sin2α)^2=1-2X16/25=-7/25