根号下则x-1>=0,x>=1
1-x>=0,x<=1
统统是成立则x=1
所以x-1=1-x=0
所以y=0+0+1/2=1/2
所以原式=1/(y-1)×√(1-y)²
=1/(y-1)×|1-y|
=1/(-1/2)×|1/2|
=-1