f(x)=3sin(2x+10)+5sin(2x+70)
=3sin(2x+10)+5sin[(2x+10)+60]
=3sin(2x+10)+5{sin(2x+10)cos60+cos(2x+10)sin60}
=3sin(2x+10)+5{(1/2)sin(2x+10)+(√3/2)cos(2x+10)}
=(11/2)sin(2x+10)+(5√3/2)cos(2x+10)
=√[(11/2)^2+(5√3/2)^2]·sin[(2x+10)+arctan((5√3/2)/(11/2))]
=7·sin[(2x+10)+arctan(5√3/11)]
∵sin[(2x+10)+arctan(5√3/11)]≤1,
∴由此可见,函数f(x)的最大值就是7