cos 2x+√3 sin2x=k+1
2sin(2x+π/6)=k+1
0<=2x<=π
π/6<=2x+π/6<=7π/6
sin相同时,2x+π/6有两个值
则π/6<=2x+π/6<=5π/6,且不等于π/2
此时a和b关于π/2对称
所以a+b=2×π/2=π