-π/2
所以0
sin²(a-b)=1-cos²(a-b)=25/169
sin(a-b)=5/13
cos²(a+b)=1-sin²(a+b)=1-9/25=16/25
cos(a+b)=4/5或-4/5
sin2a=sin(a+b+a-b)=sin(a+b)cos(a-b)+cos(a+b)sin(a-b)
sin2a=(-3/5)*12/13+(4/5)*(5/13)=-36/65+20/65=-16/65
或
sin2a=(-3/5)*12/13+(-4/5)*(5/13)=-36/65-20/65=-56/65