算术平方根有意义,x-1≥0 x≥1；1-x≥0 x≤1
x=1
y>0-0+2
y>2
y-1>0
[2/(y-1)]√(1-2y+y²)
=[2/(y-1)]√(y-1)²
=2(y-1)/(y-1)
=2