f(x)=Asin(x+a) , A=1 , x+a=2kπ+π/2 1/2=sin(π/3+a) ,π/3+a=2kπ+5π/6 a=π/2 f(x)=sin(x+π/2) 3/5=sin(a+π/2)=cosa , sina=4/5 12/13=sin(b+π/2)=cosb ,sinb=5/13 f（b-d）=sin(a-b+π/2)=cos(a-b) =cosacosb+sinasinb =56/65
f(x)=Asin(x+a) , A=1 , x+a=2kπ+π/2 1/2=sin(π/3+a) ,π/3+a=2kπ+5π/6 a=π/2 f(x)=sin(x+π/2) 3/5=sin(a+π/2)=cosa , sina=4/5 12/13=sin(b+π/2)=cosb ,sinb=5/13 f（b-d）=sin(a-b+π/2)=cos(a-b) =cosacosb+sinasinb =56/65