已知sin(π/4-α)=5/13,0

已知sin(π/4-α)=5/13,0<α<π/4,求cos2α,cos2α/cos(π/4+α)

两端平方得 [sin(π/4-a)]^2=25/169 ,
即 [1-cos(π/2-2a)]/2=25/169 ,
所以 [1-sin(2a)]/2=25/169 ,
则 sin(2a)=119/169 ,
由于0