若sin(π/4-α)=5/13,且α(0,π/2),则cos2α/cos(π/4+α）为
问题描述：
若sin(π/4-α)=5/13,且α(0,π/2),则cos2α/cos(π/4+α）为
答
cos(pai/4+α）=sin（π/4-α）=5/13,cos2α=sin2（π/4-α）=2sin（π/4-α）cos(π/4-α）=2*5/13*12/13=120/169 ,故待求式=(120/169)/(5/13)=24/13