f(3a+π/2)=2sin(1/3（3a+π/2）-π/6)=2sina=10/13,sina=5/13,cosa=12/13f(3b+2π)=2sin(1/3（3b+2π）-π/6)=2cosb=6/5,cosb=3/5,sinb=4/5cos(a+b）=cosa*cosb-sina*ainb=12/13*3/5-5/13*4/5=16/65