1/2=log(1/2)[(1/2)^(1/2)]
=log(1/2)[√(1/2)]
=log(1/2)[1/√2]
0<1/2<1
所以=log(1/2)x是减函数
log(1/2)x<=log(1/2)[1/√2]
所以x>=√2/2