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5.1 Estimating with Finite Sums
320 miles
50 ft/sec
12,000 gallons
-3
0
1
2
3
1 2 3 4
time
velocity
After 4 seconds, the object has gone 12 feet.
Consider an object moving at a constant rate of 3 ft/sec.
Since rate . time = distance:
If we draw a graph of the velocity, the distance that the object travels is equal to the area under the line.
ft3 4 sec 12 ft
sec
3t d
0
1
2
3
1 2 3 4
If the velocity is not constant,we might guess that the distance traveled is still equalto the area under the curve.
(The units work out.)
211
8V t Example:
We could estimate the area under the curve by drawing rectangles touching at their left corners.
This is called the Left-hand Rectangular Approximation Method (LRAM).
1 11
8
11
2
12
8t v
10
1 11
8
2 11
2
3 12
8Approximate area: 1 1 1 3
1 1 1 2 5 5.758 2 8 4
We could also use a Right-hand Rectangular Approximation Method (RRAM).
11
8
11
2
12
8
Approximate area: 1 1 1 31 1 2 3 7 7.75
8 2 8 4
3
0
1
2
3
1 2 3 4
211
8V t
Another approach would be to use rectangles that touch at the midpoint. This is the Midpoint Rectangular Approximation Method (MRAM).
1.031251.28125
1.78125
Approximate area:6.625
2.53125
t v
1.031250.5
1.5 1.28125
2.5 1.78125
3.5 2.531250
1
2
3
1 2 3 4
In this example there are four subintervals.As the number of subintervals increases, so does the accuracy.
211
8V t
211
8V t
Approximate area:6.65624
t v
1.007810.25
0.75 1.07031
1.25 1.19531
1.382811.75
2.25
2.75
3.25
3.75
1.63281
1.94531
2.32031
2.75781
13.31248 0.5 6.65624
width of subinterval
0
1
2
3
1 2 3 4
With 8 subintervals:
The exact answer for thisproblem is .6.6
Circumscribed rectangles are all above the curve:
0
1
2
3
1 2 3 4
0
1
2
3
1 2 3 4
Inscribed rectangles are all below the curve:
We will be learning how to find the exact area under a curve if we have the equation for the curve. Rectangular approximation methods are still useful for finding the area under a curve if we do not have the equation.
The TI-89 calculator can do these rectangular approximation problems. This is of limited usefulness, since we will learn better methods of finding the area under a curve, but you could use the calculator to check your work.
If you have the calculus tools programinstalled:
Set up the WINDOW screen as follows:
Select Calculus Tools and press Enter
Press APPS
Press F3
Press alpha and then enter: 1/ 8 ^ 2 1x
Make the Lower bound: 0Make the Upper bound: 4Make the Number of intervals: 4
Press Enter
and then 1
Note: We press alpha because the screen starts in alpha lock.