f（△x+1）-f（1）/△x=[v(x+1)-1]/x
=[v(x+1)-1]/{[v(x+1)]^2-1}
=[v(x+1)-1]/{[v(x+1)+1][v(x+1)-1]}
=1/[v(x+1)+1]
=1/2.v是根号v是根号[v(x+1)]^2=x+1，x=(x+1)-1=[v(x+1)]^2-1。