sinα=3/5,α属于(π/2,π)则cos(π/4-α)的值

sinα=3/5,α属于(π/2,π)则cos(π/4-α)的值
sinα=3/5,α属于(π/2,π)则cos(π/4-α)的值

sinα=3/5,α属于(π/2,π)则cos(π/4-α)的值
这个我答过啊.答案没有问题的,
cosα=-3/5,α属于(π/2,π)
sinα>0
又sin²α=1-cos²α=1-(-3/5)²=16/25
∴ sinα=4/5
∴ cos(α-π/4) =cosα*cos(π/4)+sinαsin(π/4) =(-3/5)*(√2/2)+(4/5)*(√2/2) =√2/10