00:01
Okay, for number 53, it's more of a conceptual problem.
00:06
We're going to try to figure out why is the left sum, left end point sum, a lower bound for the area on an increasing graph, a positive increasing graph.
00:23
So all of the function values or all of the heights are positive or above the x -axis.
00:31
And f is increasing the entire way from a to b.
00:35
So we can sketch a little picture over here to help us understand this.
00:43
We could go over here in the second quadrant, but we'll stay in the first quadrant.
00:49
Just go from a to b.
00:59
Okay.
01:00
And we have a positive function value.
01:03
And we have an increasing graph.
01:11
Okay.
01:13
Let's take a little sub -interval in here.
01:15
We can label these two other variables.
01:26
And let's just go ahead and illustrate what a left endpoint rectangle would look like, what actually happens...