00:01
All right, here they give us a fourth degree polynomial, and it has some graph, and we want to find the points on the graph where the slope of the tangent line is each of these three things.
00:11
So to find the slope of the tangent, we need the derivative.
00:14
So let's do that, f prime of x, and we're just using the power rule.
00:17
We bring that four down in front, it multiplies with a quarter to get one.
00:21
That's great.
00:21
Decrease the power by one.
00:23
Minus, same thing happens.
00:24
The three multiplies with the one -third to get one.
00:26
Decrease the power by one.
00:28
And then minus 2 goes out in front and it's x to the 1.
00:32
And now we just have to set this equal to each of these values.
00:39
So part a, we have x cubed minus x squared minus 2x equals negative 2x.
00:48
This actually means that x cubed minus x squared is zero because if we add 2x to both sides, everything else disappears.
00:54
So x squared times x minus 1 is 0.
00:58
A great idea to solve higher degree polynomial equations by factoring.
01:03
That way we don't lose any solutions.
01:05
It's very tempting to divide through by x squared, but then you would lose the x equals zero solution because either that can be zero or that can be zero.
01:12
So x equals zero or x equals one.
01:16
And then we might actually want to find the whole, they said to find points, not just x values.
01:23
So we're going to have to find f is zero, which is zero.
01:26
That's great.
01:27
And then f 1 is some fraction arithmetic.
01:33
1 4th, minus a third, minus 1.
01:36
I believe that's negative 1 .12th.
01:38
So we've got negative 13 twelfths.
01:40
So here we have the point 0 comma 0 and the point 1 comma negative 13 twelfths.
01:48
Okay, for part b, do the same thing except set this equal to 0.
01:54
X squared minus 2x equals 0...