Step by step
The question
Question: find the area under y = 3x² from x = 1 to x = 2.
1 Draw the region
Draw the curve first.
The area we want is shaded: under the curve, above the x-axis, from 1 to 2.
This area is the integral from 1 to 2.
2 The antiderivative
First the antiderivative: add 1 to each power, divide by the new power.
Now we have F(x) = x³.
No + C here: in F(b) minus F(a), the two Cs cancel.
3 Top minus bottom
The rule: put in the top number, then take away the bottom number.
Write F(x) with 1 and 2 on the bar.
Put in 2 first, then take away the same with 1.
Work out each bracket.
Careful!
Top first, then take away the bottom.
The other way gives the wrong sign.
So the area is 7 square units.
4 Check
Let's check F: its derivative is 3 x², the function we started with.
Right!
Let's look back.
First: draw the region.
Then: the antiderivative.
Then: top minus bottom.
Last: check.