00:01
Okay, in this question, we are going to discuss this function, fx being x squared.
00:07
Okay, we want to consider its concavity.
00:12
Okay, we'll show this concave.
00:17
We'll concave up.
00:19
Okay, in the graph, this concave up property is very easy because it's not very difficult for us to get the graph for this function.
00:32
It's a parabolo, right? so the graph would be like that.
00:38
Here is our x, here is our y, this is our original point 0, 0.
00:43
Okay, we say this function is concave up if and only if.
00:49
Let's choose any two points.
00:51
Let's say this is p, this is q.
00:55
Okay, the coordinate for p, let's assume it is a, a squared.
00:59
And the coordinate q is b squared.
01:03
We say this function is concave if the line segment of this straight line, i say p and q, always lies above the graph of this function.
01:17
But here we need to notice, we see the graph means the graph determined by those two points.
01:23
I mean, it's straightforward if we want to consider those two parts, those two pieces then, and those two pieces just lie, both lie above this line segment.
01:35
That means given those two points, we say we have some parts of our parabola or some part of our graph determined by those two points.
01:47
I think this red part.
01:51
We say we define the concave -r property if this is true for any two -point.
02:01
Given any two -point, the green line segment always lies above the red one.
02:12
Once the graph of a function has this property, then we say it is concave -bound.
02:18
It is true for any p.
02:20
For example, we can choose, let's say p ' here and q ' here...