a、b是銳角則a+b是0~π之間的角
因此由cosb=12/13,cos(a+b)=3/5,有sinb=5/13,sin(a+b)=4/5
1)sin(b+π/3)=sinbcos(π/3)+cosbsin(π/3)
=(5/12)*(1/2)+(12/13)(√3/2)
=(5+12√3)/26
2)cosa=cos[(a+b)-b]=cos(a+b)cosb+sin(a+b)sinb
=(3/5)(12/13)+(4/5)(5/13)=56/65
cos(2a)=2(cosa)^2-1
=2(56/65)^2-1
=2047/5625


)^2是平方2
12√3是根號3