log1/2（x-1）≥2即log1/2（x-1）≥log1/2（1/4）因为log1/2X是减函数所以0得出1
log1/2（x-1）≥2即-log2 (x-1)≥2即log2 (x-1)<=-2所以x-1<=1/4结合定义域x-1>0解得1
log1/2（x-1）≥201
