00:01
So in this question, we're asked to use the definition of the derivative to find f prime of x if f of x is equal to negative x squared plus 3x minus 5.
00:11
So first, we remember what the limit definition of the derivative is.
00:17
F prime of x is equal to the limit.
00:21
As h approaches 0 of f of x plus h minus f of x all being divided by h.
00:34
What is f of x plus h this time? f of x plus h means i go to my f of x and i replace every instance of x with the quantity of x plus h.
00:51
So that i have negative the quantity of x plus h being squared plus 3 times the quantity of x plus h minus 5.
01:06
That's f of x plus h.
01:09
Now from this, i subtract f of x.
01:13
I subtract the entire quantity of f of x, so that's got to go in parentheses.
01:21
And then this is all being divided by h.
01:27
Now, we have a bunch of algebra to do.
01:30
We're going to have to multiply things out and combine like terms.
01:36
So let's see, if i square out the x plus h quantity squared, i get xx.
01:43
Squared plus 2xh plus h squared.
01:49
Distribute the 3 plus 3x plus 3h minus 5.
01:58
Distribute the negative side plus x squared minus 3x plus 5.
02:09
All of this is being divided by each.
02:13
Now notice a few things start to cancel for me.
02:15
The negative 5 and the positive 5 cancel.
02:18
The positive 3x and the negative 3x they cancel as well.
02:25
In my next line, i'm going to maintain my limit as h approach is 0.
02:31
And i distribute the negative sign at the beginning.
02:36
Negative x squared minus 2xh, minus h squared, plus 3h, plus x squared, all being divided by h...