area under curve. calculus was historically developed to find a general method for determining the...
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IntegrationArea under Curve
Stained-glass windows
How much glass is needed?
How much glass is needed?
History
Calculus was historically developed to find a general method for determining the area of geometrical figures.
When these figures are bounded by curves, their areas cannot be determined by elementary geometry.
Integration can be applied to find such areas accurately.
Trapezoidal Rule
Also known as Trapeziod/Trapezium Rule
An approximating technique for calculating area under a curve
Works by approximating the area as a trapezium
How much glass is needed?
1Area of a trapezium height sum of parallel sides
2
2
1 1Shaded Area (1)(4 3.8) (1)(3.8 3)
2 21 1
(1)(3 2) (1)(2 0)2 2
10.8 units
Actual Area = 10.67 units2.
Trapezoidal Rule
1Area of a trapezium height sum of parallel sides
2
1Shaded Area ( ) ( )
2b a f a f b
(2, 4)
(1, 1)2y x
2
1 1Shaded Area (1)(0 1) (1)(1 4)
2 2
3 units
From diagram, clearly, it is an overestimate.
Actual Area = 2.67 units2.
Using Integration to find exact area
1
0
12
0
2
Shaded Area
2
0.5
x dx
x
unit
Using Integration to find exact area
4
0
42
0
42
0
2
Shaded Area 2
2
2
16
x dx
x
x
unit
Why?
Using rectangles to approximate the area under the line
- Show Geogebra
Using rectangles to approximate the area under the line
Dividing the area under the line into 4 strips,
We will start to approximate the area by finding the area of the rectanglesWidth of each rectangle = 0.251 1 31 1
44
1
444 2Area
1 1,
4 4
1 1,
2 2
3 3,
4 4
1
41
4
1
4
3 1
8 2actual area underestimate
1 1 3
4 2
1
44
3
8
What about n strips?
width of each rectangle =1
n
1
n
1
n
1
n
1
n
0
Find the height of each rectangle
Write down the statement for the area of each rectangle and sum them up
1
n
2
n
3
n
4
n
5......n
n
n
1......
n
n
What about n strips?
Dividing the area under the line into n strips,
1 1 11 2 3 1..
1.
n n n
nArea
n n nn n
width of each rectangle =1
n
1
n
1
n
1
n
1
n
01
n
2
n
3
n
4
n
5......n
n
n
1......
n
n
2
221 1
2 2
n n
n
n
5 5,n n
21 2 3 ...
11n
n
2
1 ( 1)
2
n n
n
What about n strips?1
2
1
2Are
na
1
n
1
n
1
n
1
n
01
n
2
n
3
n
4
n
5......n
n
n
1......
n
n
As we increase the no. of rectangles, the white triangles will be filled up by the rectangles and we will get a better approximation of the area.
,1
20As n
n
1
2Area
What about a curve?Similarly, we divide the area under the curve into n strips.
width of each rectangle =1
n
1
n
1
n
1
n
1
n
1
n
2
n
3
n
4
n
5......n
n
n
1......
n
n
Find the height of each rectangle
Write down the statement for the area of each rectangle and sum them up
What about a curve?Similarly, we divide the area under the curve into n strips.
width of each rectangle =1
n
1
n
1
n
1
n
1
n
1
n
2
n
3
n
4
n
5......n
n
n
1......
n
n
2
2,
55
n n
2 2 2 21 11 2 3 11
.1
. .n
Arn n
ean n nn nn
2 2 23
21 2 3 ... ( 11
)n
n
3
1 ( 1)(2 1)
6
n n n
n
What about a curve?
1
n
1
n
1
n
1
n
1
n
2
n
3
n
4
n
5......n
n
n
1......
n
n
2
2,
55
n n
3
1 ( 1)(2 1)
6
n n nArea
n
2
1 1, 0
2 6As n
n n
1
3Area
2
2
2 3 1
6
n n
n
2
1 1 1
3 2 6n n
Area by Integration
12
0
x dx
1
3
13
03
x
Example
Find the area under the curve
y = (x – 3)2
y = (x – 3)2
between x = 3 and x = 6
Find the area under the curve between x = 3 and x = 6
y = (x – 3)2
y = (x – 3)2
y = (x – 3)2
y = (x – 3)2
2( 3)y x
62
3
( 3)x dx63
3
2
( 3)
3
9
x
units
2 2 2 2 2 ( 1)1 2 3 4 ... ( 1)
2
n nn
2 2
2 2
( 1) 2 1
( 1) 2 1
n n n
n n n
2 2
2 2
2 2
2 2
2 2
2 1 2 1 1
3 2 2 2 1
4 3 2 3 1
5 4 2 4 1
( 1) 2 1k k k
1
2
3
4
n
n
n
n
n k
Adding both sides,
2 2
2 2
( 1) 1 2(1 2 3 ... )
( 1) 11 2 3 ...
2
k k k
k kk
2 2
2
( 1) 11 2 3 ...
2
( 1) ( 1)
2( 1)( 1 1)
2( 1)
2
k kk
k k
k k
k k
2 2 2 2 2 ( 1)1 2 3 4 ... ( 1)
2
n nn
3 3 2
3 3 2
( 1) 3 3 1
( 1) 3 3 1
n n n n
n n n n
3 3 2
3 3 2
3 3 2
3 3 2
3 3 2
2 1 3(1) 3(1) 1
3 2 3(2) 3(2) 1
4 3 3(3) 3(3) 1
5 4 3(4) 3(4) 1
( 1) 3( ) 3( ) 1k k k k
1
2
3
4
n
n
n
n
n k
2 2 2 2 ( 1)(2 1)1 2 3 ...
6
k k kk
Adding both sides,3 3 2 2 2 2( 1) 1 3(1 2 3 ... ) 3(1 2 3 ... )k k k k
2 2 2 2 3 3
32 2 2 2
32 2 2 2
2
2
2
3(1 2 3 ... ) ( 1) 1
2( 1) 2 2 3 ( 1)3(1 2 3 ... )
2
2( 1) 2( 1) 3 ( 1)(1 2 3 ... )
6
( 1)2( 1) 2 3
6
( 1) 2 4 2 2 3
6
( 1)(2 )
6( 1)(2
3 ( 1
1)
6
)
2k k k
k k k kk
k k k kk
k k k
k k k k
k k k
k k k
k k
( 1)
2
k k