00:01
As we look at this function, f of x is equal to 2 minus cosine of 3x.
00:12
And they also have this stipulation.
00:15
Let me find it real quick, that we only want to be between x equals 0 and pi over 3.
00:29
So if i zoom in on that, you can see that it's only increasing there.
00:36
And we can consider that it's always increasing because if i were to draw the derivative, it is always positive between 0 and pi over 3.
00:48
So why do we care about that? is when the derivative is positive, then f of x is increasing the entire time.
00:58
And why do we care about that so much? is because if we're always increasing or always decreasing, just make sure you're only going in one direction, then it passes the vertical line test and horizontal line test, and therefore, or so, f is 1 to 1.
01:20
There we go.
01:22
So there's a graph that kind of explains that, if anything else, what i might do is have people write out what f prime of x is actually equal to because the derivative of cosine is negative sign, so it changes the sign of it, and then you would have to multiply by three.
01:38
Oops, put that three in the wrong spot...