3.5 Integration as the Summation of Areas

3.5 Integration as the Summation of Areas

(A) Area of the region between a Curve and the x-axis.



Area of the shaded region;  A = a b y d x


(B) Area of the region between a curve and the y-axis.


Area of the shaded region;  A = a b x d y


(C) Area of the region between a curve and a straight line.


Area of the shaded region;  A = a b f ( x ) d x a b g ( x ) d x


Example 1
Find the area of the shaded region.


Solution:
Area of the shaded region = a b y d x = 0 4 ( 6 x x 2 ) d x = [ 6 x 2 2 x 3 3 ] 0 4 = [ 3 ( 4 ) 2 ( 4 ) 3 3 ] 0 = 26 2 3 unit 2


Example 2
Find the area of the shaded region.


Solution:
y = x —–(1)
x = 8yy2—–(2)
Substitute (1) into (2),
y = 8yy2
y2 – 7y = 0
y (y – 7) = 0
y = 0 or 7
From (1), x = 0 or 7
Therefore the intersection points of the curve and the straight line is (0, 0) and (7, 7).

Intersection point of the curve and y-axis is,
x = 8yy2
At y-axis, x = 0
0 = 8yy2
y (y – 8) = 0
y = 0, 8

Area of shaded region = (A1) Area of triangle + (A2) Area under the curve from y = 7 to y = 8.
= 1 2 × base × height + 7 8 x d y = 1 2 × ( 7 ) ( 7 ) + 7 8 ( 8 y y 2 ) d y = 49 2 + [ 8 y 2 2 y 3 3 ] 7 8 = 24 1 2 + [ 4 ( 8 ) 2 ( 8 ) 3 3 ] [ 4 ( 7 ) 2 ( 7 ) 3 3 ] = 24 1 2 + 85 1 3 81 2 3 = 28 1 6 unit 2

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