0:00
Right.
00:01
We're given that f prime of x is x squared, and that y equals 4x plus 7 is tangent to the graph of f.
00:11
And we want to figure out what f is.
00:13
Okay, so from this, i can take an anti -derivative.
00:17
So f of x must be x cubed over three, increase the power by one, divide by the new power, plus some constant.
00:25
Okay.
00:26
And then, um, how do we find a, tangent line.
00:33
Okay, so we have a tangent line.
00:35
This is slope four passing through.
00:40
I don't know where this is tangent, what the point of tangency is.
00:45
So i know it passes through.
00:47
I can i can write down infinitely many points.
00:50
I definitely know it passes through 0 comma 7.
00:52
That's the y intercept.
00:54
But i can write down a lot more points that it passes through.
00:57
For example, like 1 comma 11.
01:00
I just plug in the value for x.
01:01
Get a value for y, right? okay, so not entirely sure what to do with that yet, but let's use the slope four maybe.
01:10
Well, slope is f prime.
01:14
So if i set f prime of x equal to four, then that's x squared equals four.
01:23
Okay, and i like to do this by difference of squares when i can.
01:27
This makes sure i don't screw up and drop one of the solutions.
01:31
So if x minus two times x plus two equals zero, one of them must be zero.
01:36
So either x minus two equals zero and x is two or x plus two equal zero and x is negative two.
01:43
Okay, so i have two possible values for x that give me that.
01:49
Let's put those in to f of x.
01:52
So f of two is two cubed is eight over three plus c and and f of negative 2 is negative 8 over 3 plus c.
02:06
Okay.
02:07
And i guess i need to find these points then, 2 comma, and negative 2 comma, and see which of those makes sense.
02:19
So if i put in 2 for x, i get 15 for y.
02:24
If i put in negative 2 for x, i get negative 1 for y.
02:28
Okay.
02:30
So, and let's see.
02:34
It just says find a function.
02:36
So you can just choose one, but i think there's going to be two solutions...