00:01
Okay, so they give you the function, y equals sine of x minus one -third sign of x, sorry, sine of 3x, and then plus one -fifth sine, oops, minus 5x.
00:22
And we only care about values for x equals 0 to x equals pi.
00:30
So i can zoom in.
00:34
We only care about these values from that, the y -axis to x equals pi.
00:40
So if you're just looking at this graph, i think it's pretty clear that's concave down first, and then it goes to concave up, and then it goes to concave down, then concave up, and then concave down.
00:52
And what's nice about using a computer algebra system like this one is i can just write f prime of x.
00:58
And this is showing me the derivative.
01:04
Let me hide these things.
01:06
The derivative.
01:08
And because it's increasing, because f is increasing, that means the derivative is positive.
01:15
You can see it's positive this entire time.
01:18
Now, it's actually zero here because you have a horizontal tangent at, what's that, pi over six.
01:26
But it's still increasing, still increasing until this value.
01:31
Pi over 2.
01:32
Why? because at pi over 2, it changes from, f changes from increasing to decreasing, which means the graph of the derivative needs to be below the x -axis...