f'(x) =lim(h->0)[ f(x+h) -f(x)] /h
=lim(h->0)[ (x+h)^2-1 - x^2 +1 )] /h
=lim(h->0)( 2xh +h^2 ) /h
=lim(h->0)( 2x +h )
=2xh后面的一杠是什么意思？