送交者: HunHunSheng 于 2007-07-10, 11:35:54:
回答: 极限好象需要级数作为对象研究? 由 Nixrreg 于 2007-07-10, 11:30:53:
f'(x)=lim ((x+d)^2-x^2)/d=(2xd+d^2)/d=2x+d
Very intuitively if d approach zero, 2x+d approach 2x.
(But strictly proving it need strict definition of limit)
So the derivative of x^2 is 2x