cos(α+π/2)=-sinα=-根号[1-cos(α)^2]=-根号[1-4/25]=-根号21/5
cosα=2/5,且α是第一象限角,cos(α+π/2)=-sinα=-√(1-cos²α)=-√21/5
cos(α+π/2)=-sina=负五分之根号21
