f(x)=3sin(x+20°)+5sin(x+80°)
=3sin(x+20°)+5sin[(x+20°）+60°)]
=3sin(x+20°)+5sin(x+20°)cos60+5cos(x+20)sin60
=11/2 sin(x+20)+5根号3/2 cos(x+20)
=7sin(x+20+b)
最大值7