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Karnataka Board PUCPUC Science 2nd PUC Class 12

Properties of Indefinite Integral

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Estimated time: 13 minutes
CBSE: Class 12

Properties of Indefinite Integrals

1.Inverse of Differentiation

\[\frac{d}{dx}\left(\int f(x) dx\right) = f(x)\]

This shows that differentiation and indefinite integration are inverse processes.

\[ \int f'(x)\, dx = f(x) + C \]

Thus, differentiation and integration are inverse processes of each other.

2. Same Derivative Implies Same Family of Antiderivatives

If \[\frac{d}{dx}[F(x)] = \frac{d}{dx}[G(x)]\]

then \[F(x) = G(x) + C\]

Thus, two indefinite integrals having the same derivative represent the same family of curves and are considered equivalent.

3. Sum Rule

\[\int (f(x) + g(x)) dx = \int f(x) dx + \int g(x) dx\]

This means integration is distributive over addition.

4. Difference Rule

\[\int (f(x) - g(x)) dx = \int f(x) dx - \int g(x) dx\]

This is the corresponding rule for subtraction.

5. Constant Multiple Rule

\[\int kf(x) dx = k \int f(x) dx\] where k is a constant.

6. General Linearity Rule

\[ \int [k_1 f_1(x) + k_2 f_2(x) + \cdots + k_n f_n(x)]\, dx = k_1 \int f_1(x)\, dx + k_2 \int f_2(x)\, dx + \cdots + k_n \int f_n(x)\, dx \]

where \[k_1, k_2, \ldots, k_n\] are real constants.

CBSE: Class 12

Integration by Method of Inspection

Method of Inspection:

To find an antiderivative of a function, we may look for a known function whose derivative is the given function. This is called integration by the method of inspection.

Example

Find an antiderivative of \[ \cos 2x. \]

Solution:

We know \[ \frac{d}{dx}(\sin 2x) = 2\cos 2x. \]

Therefore, \[ \int \cos 2x\, dx = \frac{1}{2}\sin 2x + C \]

CBSE: Class 12

Example 1

Find the following integrals:

  1. \[\int (\sin x + \cos x) dx\]
  2. \[\int \text{cosec } x (\text{cosec } x + \cot x) dx\]
  3. \[\int \frac{1 - \sin x}{\cos^2 x} dx\]

Solution:

(i) We have

\[\int (\sin x + \cos x) dx = \int \sin x dx + \int \cos x dx\]

\[= -\cos x + \sin x + \text{C}\]

(ii) We have

\[\int \text{cosec } x (\text{cosec } x + \cot x) dx = \int \text{cosec}^2 x dx + \int \text{cosec } x \cot x dx\]

\[= -\cot x - \text{cosec } x + \text{C}\]

(iii) We have

\[\int \frac{1 - \sin x}{\cos^2 x} dx = \int \frac{1}{\cos^2 x} dx - \int \frac{\sin x}{\cos^2 x} dx\]

\[= \int \sec^2 x dx - \int \tan x \sec x dx\]

\[= \tan x - \sec x + \text{C}\]

CBSE: Class 12

Example 2

Find the anti derivative F of \[f\] defined by \[f(x) = 4x^3 - 6\], where \[\text{F}(0) = 3\]

Solution: One anti derivative of \[f(x)\] is \[x^4 - 6x\] since

\[\frac{d}{dx} (x^4 - 6x) = 4x^3 - 6\]

Therefore, the anti-derivative F is given by

\[\text{F}(x) = x^4 - 6x + \text{C}\], where C is constant.

Given that \[\text{F}(0) = 3\], which gives,

\[3 = 0 - 6 \times 0 + \text{C}\] or \[\text{C} = 3\]

Hence, the required anti-derivative is the unique function F defined by

\[\text{F}(x) = x^{4} - 6x + 3\].

CBSE: Class 12

Key Points: Properties of Indefinite Integrals

Property Formula
Reverse of differentiation \[\frac{d}{dx}\left(\int f(x) dx\right) = f(x)\]
Same derivative \[F'(x) = G'(x) \Rightarrow F(x) = G(x) + C\]
Sum rule \[\int (f + g)dx = \int fdx + \int gdx\]
Difference rule \[\int (f - g)dx = \int fdx - \int gdx\]
Constant multiple rule \[\int k f(x)dx = k \int f(x)dx\]
General linearity \[\int (kf \pm lg)dx = k \int fdx \pm l \int gdx\]

Test Yourself

Shaalaa.com | Integrals part 5 (Properties of indefinite integrals)

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