Step by step
The question
Question: estimate the area under y = 2x + 1 from x = 0 to x = 4, with 4 left rectangles.
A rectangle's area is easy: width times height.
So we fill the area under the curve with rectangles, and add them up.
1 The width
Each rectangle has the same width.
We call it Δx, "delta x".
The whole width, 4 minus 0, cut into 4 equal parts.
So each rectangle is 1 wide.
2 The heights
The cuts are every 1: 0, 1, 2, 3, 4.
Left rectangles: each height is at the left end.
So we use every cut but the last.
4 rectangles, 4 heights: 0, 1, 2, 3.
Careful!
4 rectangles have 4 heights, not 5.
The last cut, 4, is not used.
Now the heights: put each x into f.
f of 0 is 1.
The others the same way.
3 Draw the rectangles
Let's draw it.
The 4 rectangles: each one 1 wide, with its height at the green dot.
4 Add the rectangles
Every rectangle has the same width, so add the heights, then times the width.
The heights add up to 16.
So the area is about 16 square units.
5 Check
Let's check with the exact area: the integral from 0 to 4.
It is 20.
Our estimate is smaller.
That makes sense: under a rising curve, left rectangles stay below it.
The gaps are the missing area.
Right!
Let's look back.
First: the width.
Then: the heights.
Then: draw the rectangles.
Then: add the rectangles.
Last: check.