已知sin(π\4-a)=5\13,0

已知sin(π\4-a)=5\13,0
已知sin(π\4-a)=5\13,0

已知sin(π\4-a)=5\13,0
0< π/4-a

10/13
令TT/4-a=X a=TT/4-x 2a=TT/2-2X 0<2asinx=5/13
cos2a/cos(TT/4-a)=cos(TT/2-2x)/cosx=sin2x/cosx=2sinx=10/13

0<π/4-a<π/4
sin（π/4-a）=5/13;cos(π/4-a)=12/13
cos2a/cos(π/4-a)=sin(π/2-2a)/cos(π/4-a)=2sin(π/4-a)=10/13