00:01
Okay, in this question, we are given the function fx is equal to x squared plus 21x.
00:10
We want to use the definition of the derivative to actually compute its derivative.
00:17
Okay, first notice the domain of f is the real line.
00:21
So we want to compute its derivative for any real number.
00:29
Okay, so for any fixed number x, which is a real number, we want to confuse this limit when delta x goes to 0.
00:41
To avoid confusion, we just use delta.
00:46
The limit when delta goes to 0, f minus f over delta.
00:54
Okay, the expression of f of x when it is made can be written as z -mean -twin -delta equals to zero, here we have x plus delta squared plus twenty -one x plus delta minus x squared minus twenty -one x over delta...