x∈[π/4,3π/4],2x∈[π/2,3π/2],2X+π/6∈[π/2+π/6,3π/2+π/6]
根据sinX的图象得知,最小值为 -1,最大值为 sin(π/2+π/6)=cos(π/6)=√3/2
所以答案成立