f(x)=Asin(x+ψ) ,A=1 ,
x+ψ=2kπ+π/2
1/2=sin(π/3+ψ) ,π/3+ψ=2kπ+5π/6
ψ=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（a-b）=sin(a-b+π/2)=cos(a-b)
=cosacosb+sinasinb
=56/65
已知函数f(x)=Asin(x+ψ)(A〉0,0
f(x)=Asin(x+ψ) ,A=1 ,
x+ψ=2kπ+π/2
1/2=sin(π/3+ψ) ,π/3+ψ=2kπ+5π/6
ψ=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（a-b）=sin(a-b+π/2)=cos(a-b)
=cosacosb+sinasinb
=56/65