化简求值:已知sin(pai/4-a)=5/13,a属于(0,pai/4)求:cos2a/cos(pai/4+a)

cos(π/4+a)=sin[π/2-(π/4+a)]=sin(π/4-a)=5/13
cos(π/4-a)=根号(1-sin²(π/4-a))=12/13
sin(π/2-2a)
=sin(2×(π/4-a))
=2sin(π/4-a)cos(π/4-a)
=2×(5/13)×(12/13)
=120/169
cos(2a)=sin(π/2-2a)=120/169
cos(2a)/cos(π/4+a)
=(120/169)/(5/13)
=24/13
a属于(0,π/4)
π/4-a属于(0,π/4)
cos(π/4-a)的值是正的～

no