用导数定义求导

Question

f（x）=10x^2,求在x=-1处导数值，题目要求按导数定义。

故设变量为h，则当h趋于0时，函数化简为-20+10/h,但当h趋于0时，1/h为无穷大啊
没看懂一楼的话

Answer 1

对比下:
f(x)=10x²在x=-1处的导数值为
lim[h→0] [f(x+h)-f(x)]/h,在x=-1
=lim[h→0] [10(-1+h)²-10(-1)²]/h
=lim[h→0] [10(h²-2h+1)-10]/h
=lim[h→0] (10h²-20h)/h
=lim[h→0] (10h-20)
=-20