Step by step
The question
Question: find the average value of f(x) = x + 3 on [0, 6], and the number c where f(c) is the average.
1 The formula
The average value: the area under the curve, divided by the width of the interval.
Here the width is 6 minus 0: 6.
Think of it as the height of a rectangle with the same area as under the curve.
2 The integral
The antiderivative: add 1 to each power, divide by the new power.
Now we have x² over 2 + 3x.
Put in 6, then take away the same with 0.
That is 36 minus 0: 36.
3 Divide by the width
Divide by the width, 6.
The average value is 6.
Careful!
36 is the area.
To get the average height, divide it by the width, 6.
4 Where f(c) is the average
Now find c where f is exactly the average.
Take away 3 on both sides.
Divide by 1: c = 3.
5 Check
Let's check: a rectangle 6 high and 6 wide has area 36: the same as the integral.
Right!
In a picture:
the area under the curve,
and the rectangle at the average height.
The same area.
Let's look back.
First: the formula.
Then: the integral.
Then: divide by the width.
Then: where f(c) is the average.
Last: check.