f '(x0) = lim [f (x0 + △ x)-f (x0)] / △ x
f (x0 + △ x)-f (x0)] is a function from x0,to x0 + △ x = △ y amount of change
△ x approaches 0,f (x0 + △ x)-f (x0)] do not approach infinity
If △ x tends to 0 when it has a limit,that is,the derivative of x0
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Still can not quite understand,but I know you said is right.Again explain?Secant slope limit fertility problems I see,but why use this formula that division,I think is not perfect.You say?first,Thank you very much.I agree with you .I also think this formula is not perfect.