cos(α-π)
=-cosα
=-1/2
cosα=1/2
∵α是第四象限角
∴sinα=-√3/2
cos(π/4+α)
=cosπ/4cosα-sinπ/4sinα
=√2/2*1/2-√2/2*(-√3/2)
=√2/4+√6/4
=(√2+√6)/4