f(x) = 2sin(2x+π/6)x属于【0，π/2】2x属于【0，π】2x+π/6属于【π/6，7π/6】2x+π/6 = 7π/6时有最小值 -12x+π/6 = π/2时有最大值2值域【-1，2】
