求问 这道题怎么解

let
y= x-1
lim(x->1) tan[2(x-1)]/(√x -1)
=lim(y->1) tan(2y)/[√(1+y) -1]
=lim(y->1) (2y)/[1+(1/2)y -1]
=lim(y->1) (2y)/[(1/2)y ]
=4