UNIT 4 | unit 4 multivariable calculus integration

MATHS I

UNIT 4

MULTIVARIABLE CALCULUS (INTEGRATION)

4.1 Double integrals

Double integral –

Before studying about multiple integrals, first let’s go through the definition of definition of definite integrals for function of single variable.

As we know, the integral

Where is belongs to the limit a ≤ x ≤ b

This integral can be written as follows-

Now suppose we have a function f(x , y) of two variables x and y in two dimensional finite region R in xy-plane.

Then the double integration over region R can be evaluated by two successive integration

Evaluation of double integrals-

If A is described as

Then,

]dx

Let do some examples to understand more about double integration-

Example-1: Evaluate , where dA is the small area in xy-plane.

Sol. Let , I =

= 84 sq. Unit.

Which is the required area.

Example-2: Evaluate

Sol. Let us suppose the integral is I,

I =

Put c = 1 – x in I, we get

I =

Suppose, y = ct

Then dy = c

Now we get,

I =

As we know that by beta function,

Which gives,

Now put the value of c, we get

Example-3: Evaluate the following double integral,

Sol. Let,

I =

On solving the integral, we get

Double Integral over Rectangular and general regions

Consider a function f (x, y) defined in the finite region R of the x-y plane. Divide R into n elementary areas A1, A2,…,An. Let (xr, yr) be any point within the rth elementary are Ar

Fig. 6.1

 f (x, y) dA = f (xr, yr)  A

Evaluation of Double Integral when limits of Integration are given (Cartesian Form).

Ex. 1: Evaluate ey/x dy dx.

Soln.:

Given: I = ey/x dy dx

Here limits of inner integral are functions of y therefore integrate w.r.t y,

I = dx

I =

= =

ey/x dy dx =

Ex. 2: Evaluate xy (1 – x –y) dx dy.

Soln.:

Given: I = xy (1 – x –y) dx dy.

Here the limits of inner integration are functions of y therefore first integrate w.r.t y.

I = xdx

Put 1 – x = a (constant for inner integral)

 I = xdx

Put y = at  dy = a dt

y	0	a
t	0	1

 I = xdx

I = xa dx 

I =  x (1 – x) dx =  (x– x4/3) dx

I =

I = =

 xy (1 – x –y) dx dy =

Ex. 3 : Evaluate

Soln. :

Let, I =

Here limits for both x and y are constants, the integral can be evaluated first w.r.t any of the variables x or y.

I = dy

I =

= 

Ex. 4: Evaluate e–x2 (1 + y2) x dx dy.

Soln. :

Let I = e–x2 (1 + y2)  x dy = dy e–x2 (1 + y2)  x dy

= dy e– x2 (1 + y2)  dx

= dy  [∵ f (x)  ef(x) dx = ef(x) ]

= (–1) dy (∵ e– = 0)

= = =

 e–x2 (1 + y2) xdx dy =

NOTES:

Type II: Evaluation of Double Integral when region of Integration is provided (Cartesian form)

Ex.1: Evaluate  y dx dy over the area bounded by x= 0 y = and x + y = 2 in the first quadrant

Soln. :

The area bounded by y = x2 (parabola) and x + y = 2 is as shown in Fig.6.2

The point of intersection of y = x2 and x + y = 2.

x + x2 = 2 x2 + x – 2 = 0

 x = 1, – 2

At x = 1, y = 1 and at x = –2, y = 4

Fig. 6.2

 (1, 1) is the point of intersection in Ist quadrant. Take a vertical strip SR, Along SR x constant and y varies from S to R i.e. y = x2 to y = 2 – x.

Now slide strip SR, keeping IIel to y-axis, therefore y constant and x varies from x = 0 to x = 1.

 I =

= (4 – 4x + – ) dx

= = 

 I = 16/15

Ex. 2 : Evaluate  over x  1, y 

Soln. :

1a1 Let I =  over x  1, y 

The region bounded by x  1 and y 

Is as shown in Fig. 6.3.

Fig. 6.3

Take a vertical strip along strip x constant and y varies from y =

To y = . Now slide strip throughout region keeping parallel to y-axis. Therefore y constant and x varies from x = 1 to x = .

I =

= [ ∵ dx = tan–1 (x/a)]

= =

= – = (0 – 1)

 I =

Ex. 3 : Evaluate  (+ ) dx dy through the area enclosed by the curves y = 4x, x + y = 3 and y =0, y = 2.

Soln. :

Let I =  (+ ) dx dy

The area enclosed by the curves y = 4x, x + y =3, y = 0 and y = 2 is as shown in Fig. 6.4.

(find the point of intersection of x + y = 3 and y = 4x)

Fig. 6.4

Take a horizontal strip SR, along SR y constant and x varies from x = to x = 3 – y. Now slide strip keeping IIel to x axis therefore x constant and y varies from y = 0 to y = 2.

I = dy (+ ) dx

= +dy

 I =

= + – 6 + 18

 I =

4.2 Change of order of integration

Ex. 1 : Change the order of integration for the integral and evaluate the same with reversed order of integration.

Soln. :

Given, I = …(1)

In the given integration, limits are

y = , y = 2a – x and x = 0, x = a

The region bounded by x2 = ay, x + y = 2a Fig.6.5

And x = 0, x = a is as shown in Fig. 6.5

Here we have to change order of Integration. Given the strip is vertical.

Now take horizontal strip SR.

To take total region, Divide region into two parts by taking line y = a.

1 st Region :

Along strip, y constant and x varies from x = 0 to x = 2a – y. Slide strip IIel to x-axis therefore y varies from y = a to y = 2a.

 I1 = dy xy dx …(2)

2nd Region :

Along strip, y constant and x varies from x = 0 to x = . Slide strip IIel to x-axis therefore x-varies from y = 0 to y = a.

 I2 = dy xy dx …(3)

From Equation (1), (2) and (3),

 = dy xy dx + dy xy dx

= + y dy

=dy + (ay) dy = y (4a2 – 4ay + y2) dy + ay2 dy

= (4a2 y – 4ay2 + y3) dy + y2 dy

= +

= a4

Ex. 2 : Evaluate

I =

Soln. :

Given : I = …(1)

In the given integration, limits are

x = 0, x = a, y = 0, y =

The bounded region is as shown in Fig. **.

In the given, strip is vertical. Now take horizontal strip SR. Along strip y constant and x varies from x = 0 to

x = . Slide strip IIel to X-axis therefore y varies from y = 0 to y = a.

 I = dy

Put a2 – y2 = b2

 I =

= =

= dy = dy

= [∵ a = a loge]

 I = dy =

Ex.3 : Express as single integral and evaluate dy dx + dy dx.

Soln. :

Given : I = dy dx + dy dx

I = I1 + I2

The limits of region of integration I1 are
x = – ; x = and y = 0, y = 1 and I2 are x = – 1,
x = 1 and y = 1, y = 3.

The region of integration are as shown in Fig. 6.7

To consider the complete region take a vertical strip SR along the strip y varies from y = x2 to
y = 3 and x varies from x = –1 to x = 1. Fig. 6.7

I =

4.3 Change of Variables from Cartesian to plane polar coordinates

Evaluation of Double Integral by Changing Cartesian to Polar co-ordinates (when limits are given).

Ex. 1 : Evaluate

Soln. :

The region of integration bounded by

y = 0, y = and x = 0, x = 1

y =  x2 + y2 = x

The region bounded by these is as shown in Fig. 6.8.

Convert the integration in polar co-ordinates by using x = r cos , y = r sin  and dx dy = r dr d

 x2 + y2 = x becomes r = cos 

y = 0 becomes r sin  = 0  = 0

x = 0 becomes r cos  = 0  =

And x = 1 becomes r = sec 

Take a radial strip SR with angular thickness , Along strip  constant and r varies from r = 0 to r = cos . Turning strip throughout region therefore  varies from = 0 to  =

 I = r dr d

= 4 cos  sin  d r dr

= 4 cos  sin  d [–]

= – 2 cos  sin  [+1] d

= – 2 [cos  sin  – cos  sin ] d

= –2 + 2 cos  sin  d

= – + 2

= + 1 =

Ex. 2: Sketch the area of double integration and evaluate

dxdy

Soln. :

Let I = dxdy

The region of integration is bounded by the curves

x = y, x = and y = 0, y = Fig. 6.9

i.e. x = y, x2 + y2 = a2 and y = 0, y =

The region bounded by these is as shown in Fig. 6.9.

The point of intersection of x = y and x2 + y2 = a2 is x =

Convert given integration in polar co-ordinates by using polar transformation x = r cos , y = r sin  and dx dy = r dr d

 x = y gives r cos  = r sin  tan  = 1  =

x2 + y2 = a2  r2 = a2 r =  a

y = 0 gives r sin  = 0  = 0.

y = gives r sin  =  r =  cosec 

Take a radial strip SR, along SR  constant and r varies from r = 0 to r = a. Turning this strip throughout region therefore  varies from  = 0 to  =

 I = log r2 r dr d = 2 d r log r dr

I =

= 2 d

= 2 []

I = [/4] =

Evaluation of Double Integral when region of Integration is provided (Polar form)

Ex. 1 : Evaluate  r4 cos3 dr d over the interior of the circle r = 2a cos 

Soln. :

The region of the integration is as shown in Fig. 6.10.Take a radial strip SR, along strip  constant and r varies from r = 0 to r = 2a cos. Now turning this strip throughout region therefore  varies from  = to  =

 I = r4 cos3 dr d

= cos3 d

= cos3 cos5 d

= cos8 d Fig. 6.10

=  2 

I =

 r4 cos3 dr d =

Ex. 2 : Evaluate  r sin  dr d over the cardioid r = a (1 – cos ) above the initial line.

Soln. :

The cardioid r = a (1 – cos ) is as shown in Fig. 6.11. The region of the integration is above the initial line.

Take a radial strip SR, along strip  constant and r Varies from r = 0 to r = a (1 – cos ).

11a New turning the strip throughout region therefore  varies from  = 0 to  = .

 I = r sin  dr d

= sin  d

Fig.6.11

= sin  [a2 (1 – cos )2]

I = (sin  – 2 sin  cos  + sin ) d

=  2 (sin  – sin2  + sin ) d

= +

I = a2= a2

I =

NOTES:

Area enclosed by plane curves expressed in Cartesian coordinates:

Y (x,y) P Q (x+dx,y+dy) y=f(x)

V U

G H dxdy

A R S B

x=a y=0 x=b X

Consider the area enclosed by the curves

And and the ordinates

Area =

Consider the area enclosed by the curves

And and the ordinates

Area =

Area enclosed by plane curves expressed in Polar coordinates:

Consider the area enclosed by the polar curves

And and the line

Area =

Ex.1: Find the area between the curves and its asymptote.

Solution: The curve is symmetrical about x axis not passing through origin. Also is the asymptote to the curve and intersect x axis at for curve doesn’t exists. And for and. Because of symmetry

Put

x	0	2a
t	0	1

Ex: 2: Show that the area of curve is

Solution: The curve is symmetrical about Y - axis passing through origin and there exists a cusp at origin. It intersects Y-axis at (0 , 0) , (0 , 2a) .

Also .

Putting

Ex.3: Find the Area included between the two cardioids

Solution: - Area =

Ex.4: Find the Area common to the two circle

Solution: - By converting the given circle into polar form we get

Area =

4.4 Triple integrals

Definition: Let f(x,y,z) be a function which is continuous at every point of the finite region (Volume V) of three dimensional space. Divide the region V into n sub regions of respective volumes. Let () be a point in the sub region then the sum: