Chapter 7 Integrals EXERCISE 7.1

EXERCISE 7.1

Find an anti derivative (or integral) of the following functions by the method of inspection.

1. $\sin 2 x$

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Solution

The anti derivative of $\sin 2 x$ is a function of $x$ whose derivative is $\sin 2 x$. It is known that,

$\frac{d}{d x}(\cos 2 x)=-2 \sin 2 x$

$\Rightarrow \sin 2 x=-\frac{1}{2} \frac{d}{d x}(\cos 2 x)$

$\therefore \sin 2 x=\frac{d}{d x}(-\frac{1}{2} \cos 2 x)$

Therefore, the anti derivative of $\sin 2 x$ is $-\frac{1}{2} \cos 2 x$

2. $\cos 3 x$

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Solution

The anti derivative of $\cos 3 x$ is a function of $x$ whose derivative is $\cos 3 x$.

It is known that,

$\frac{d}{d x}(\sin 3 x)=3 \cos 3 x$

$\Rightarrow \cos 3 x=\frac{1}{3} \frac{d}{d x}(\sin 3 x)$

$\therefore \cos 3 x=\frac{d}{d x}(\frac{1}{3} \sin 3 x)$

Therefore, the anti derivative of $\cos 3 x$ is $\frac{1}{3} \sin 3 x$.

3. $e^{2 x}$

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Solution

The anti derivative of $e^{2 x}$ is the function of $x$ whose derivative is $e^{2 x}$.

It is known that, $\frac{d}{d x}(e^{2 x})=2 e^{2 x}$

$\Rightarrow e^{2 x}=\frac{1}{2} \frac{d}{d x}(e^{2 x})$

$\therefore e^{2 x}=\frac{d}{d x}(\frac{1}{2} e^{2 x})$

Therefore, the anti derivative of $e^{2 x}$ is $\frac{1}{2} e^{2 x}$.

4. $(a x+b)^{2}$

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Solution

The anti derivative of $(a x+b)^{2}$ is the function of $x$ whose derivative is $(a x+b)^{2}$.

It is known that,

$\frac{d}{d x}(a x+b)^{3}=3 a(a x+b)^{2}$

$\Rightarrow(a x+b)^{2}=\frac{1}{3 a} \frac{d}{d x}(a x+b)^{3}$

$\therefore(a x+b)^{2}=\frac{d}{d x}(\frac{1}{3 a}(a x+b)^{3})$

Therefore, the anti derivative of $(a x+b)^{2}$ is $\frac{1}{3 a}(a x+b)^{3}$.

5. $\sin 2 x-4 e^{3 x}$

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Solution

The anti derivative of $(\sin 2 x-4 e^{3 x})$ is the function of $x$ whose derivative is $(\sin 2 x-4 e^{3 x})$.

It is known that, $\frac{d}{d x}(-\frac{1}{2} \cos 2 x-\frac{4}{3} e^{3 x})=\sin 2 x-4 e^{3 x}$

Therefore, the anti derivative of $(\sin 2 x-4 e^{3 x})$ is $(-\frac{1}{2} \cos 2 x-\frac{4}{3} e^{3 x})$.

Find the following integrals in Exercises 6 to 20:

6. $\int(4 e^{3 x}+1) d x$

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Solution

$ \begin{aligned} & \int(4 e^{3 x}+1) d x \\ & =4 \int e^{3 x} d x+\int 1 d x \\ & =4(\frac{e^{3 x}}{3})+x+C \\ & =\frac{4}{3} e^{3 x}+x+C \end{aligned} $

7. $\int x^{2}(1-\frac{1}{x^{2}}) d x$

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Solution

$\int x^{2}(1-\frac{1}{x^{2}}) d x$

$=\int(x^{2}-1) d x$

$=\int x^{2} d x-\int 1 d x$

$=\frac{x^{3}}{3}-x+C$

8. $\int(a x^{2}+b x+c) d x$

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Solution

$\int(a x^{2}+b x+c) d x$

$=a \int x^{2} d x+b \int x d x+c \int 1 \cdot d x$

$=a(\frac{x^{3}}{3})+b(\frac{x^{2}}{2})+c x+C$

$=\frac{a x^{3}}{3}+\frac{b x^{2}}{2}+c x+C$

9. $\int(2 x^{2}+e^{x}) d x$

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Solution

$\int(2 x^{2}+e^{x}) d x$

$=2 \int x^{2} d x+\int e^{x} d x$

$=2(\frac{x^{3}}{3})+e^{x}+C$

$=\frac{2}{3} x^{3}+e^{x}+C$

10. $\int(\sqrt{x}-\frac{1}{\sqrt{x}})^{2} d x$

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Solution

$ \begin{aligned} & \int(\sqrt{x}-\frac{1}{\sqrt{x}})^{2} d x \\ & =\int(x+\frac{1}{x}-2) d x \\ & =\int x d x+\int \frac{1}{x} d x-2 \int 1 \cdot d x \\ & =\frac{x^{2}}{2}+\log |x|-2 x+C \end{aligned} $

11. $\int \frac{x^{3}+5 x^{2}-4}{x^{2}} d x$

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Solution

$ \begin{aligned} & \int \frac{x^{3}+5 x^{2}-4}{x^{2}} d x \\ & =\int(x+5-4 x^{-2}) d x \\ & =\int x d x+5 \int 1 \cdot d x-4 \int x^{-2} d x \\ & =\frac{x^{2}}{2}+5 x-4(\frac{x^{-1}}{-1})+C \\ & =\frac{x^{2}}{2}+5 x+\frac{4}{x}+C \end{aligned} $

12. $\int \frac{x^{3}+3 x+4}{\sqrt{x}} d x$

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Solution

$ \begin{aligned} & \int \frac{x^{3}+3 x+4}{\sqrt{x}} d x \\ & =\int(x^{\frac{5}{2}}+3 x^{\frac{1}{2}}+4 x^{-\frac{1}{2}}) d x \\ & =\frac{x^{\frac{7}{2}}}{\frac{7}{2}}+\frac{3(x^{\frac{3}{2}})}{\frac{3}{2}}+\frac{4(x^{\frac{1}{2}})}{\frac{1}{2}}+C \\ & =\frac{2}{7} x^{\frac{7}{2}}+2 x^{\frac{3}{2}}+8 x^{\frac{1}{2}}+C \\ & =\frac{2}{7} x^{\frac{7}{2}}+2 x^{\frac{3}{2}}+8 \sqrt{x}+C \end{aligned} $

13. $\int \frac{x^{3}-x^{2}+x-1}{x-1} d x$

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Solution

$\int \frac{x^{3}-x^{2}+x-1}{x-1} d x$

On dividing, we obtain

$=\int(x^{2}+1) d x$

$=\int x^{2} d x+\int 1 d x$

$=\frac{x^{3}}{3}+x+C$

14. $\int(1-x) \sqrt{x} d x$

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Solution

$\int(1-x) \sqrt{x} d x$

$=\int(\sqrt{x}-x^{\frac{3}{2}}) d x$

$=\int x^{\frac{1}{2}} d x-\int x^{\frac{3}{2}} d x$

$=\frac{x^{\frac{3}{2}}}{3}-\frac{x^{\frac{5}{2}}}{5}+C$

$=\frac{2}{3} x^{\frac{3}{2}}-\frac{2}{5} x^{\frac{5}{2}}+C$

15. $\int \sqrt{x}(3 x^{2}+2 x+3) d x$

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Solution

$ \int \sqrt{x}(3 x^{2}+2 x+3) d x $

$=\int(3 x^{\frac{5}{2}}+2 x^{\frac{3}{2}}+3 x^{\frac{1}{2}}) d x$

$=3 \int x^{\frac{5}{2}} d x+2 \int x^{\frac{3}{2}} d x+3 \int x^{\frac{1}{2}} d x$

$=3(\frac{x^{\frac{7}{2}}}{\frac{7}{2}})+2(\frac{x^{\frac{5}{2}}}{\frac{5}{2}})+3 \frac{(x^{\frac{3}{2}})}{\frac{3}{2}}+C$

$=\frac{6}{7} x^{\frac{7}{2}}+\frac{4}{5} x^{\frac{5}{2}}+2 x^{\frac{3}{2}}+C$

16. $\int\left(2 x-3 \cos x+e^{x}\right) d x$

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Solution

$\int(2 x-3 \cos x+e^{x}) d x$

$=2 \int x d x-3 \int \cos x d x+\int e^{x} d x$

$=\frac{2 x^{2}}{2}-3(\sin x)+e^{x}+C$

$=x^{2}-3 \sin x+e^{x}+C$

17. $\int(2 x^{2}-3 \sin x+5 \sqrt{x}) d x$

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Solution

$\int(2 x^{2}-3 \sin x+5 \sqrt{x}) d x$ $=2 \int x^{2} d x-3 \int \sin x d x+5 \int x^{\frac{1}{2}} d x$

$=\frac{2 x^{3}}{3}-3(-\cos x)+5(\frac{x^{\frac{3}{2}}}{\frac{3}{2}})+C$

$=\frac{2}{3} x^{3}+3 \cos x+\frac{10}{3} x^{\frac{3}{2}}+C$

18. $\int \sec x(\sec x+\tan x) d x$

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Solution

$\int \sec x(\sec x+\tan x) d x$

$=\int(\sec ^{2} x+\sec x \tan x) d x$

$=\int \sec ^{2} x d x+\int \sec x \tan x d x$

$=\tan x+\sec x+C$

19. $\int \frac{\sec ^{2} x}{cosec^{2} x} d x \quad$

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Solution

$\int \frac{\sec ^{2} x}{cosec 2} d x$

$ \begin{aligned} & =\int \frac{\frac{1}{\cos ^{2} x}}{\frac{1}{\sin ^{2} x}} d x \\ & =\int \frac{\sin ^{2} x}{\cos ^{2} x} d x \\ & =\int \tan ^{2} x d x \\ & =\int(\sec ^{2} x-1) d x \\ & =\int \sec ^{2} x d x-\int 1 d x \\ & =\tan x-x+C \end{aligned} $

20. $\int \frac{2-3 \sin x}{\cos ^{2} x} d x$.

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Solution

$\int \frac{2-3 \sin x}{\cos ^{2} x} d x$

$=\int(\frac{2}{\cos ^{2} x}-\frac{3 \sin x}{\cos ^{2} x}) d x$

$=\int 2 \sec ^{2} x d x-3 \int \tan x \sec x d x$

$=2 \tan x-3 \sec x+C$

Choose the correct answer in Exercises 21 and 22.

21. The anti derivative of $(\sqrt{x}+\frac{1}{\sqrt{x}})$ equals

(A) $\frac{1}{3} x^{\frac{1}{3}}+2 x^{\frac{1}{2}}+C$

(B) $\frac{2}{3} x^{\frac{2}{3}}+\frac{1}{2} x^{2}+C$

(C) $\frac{2}{3} x^{\frac{3}{2}}+2 x^{\frac{1}{2}}+C$

(D) $\frac{3}{2} x^{\frac{3}{2}}+\frac{1}{2} x^{\frac{1}{2}}+C$

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Solution

$ (\sqrt{x}+\frac{1}{\sqrt{x}}) d x $

$=\int x^{\frac{1}{2}} d x+\int x^{-\frac{1}{2}} d x$

$=\frac{x^{\frac{3}{2}}}{3}+\frac{x^{\frac{1}{2}}}{1}+C$

2

$=\frac{2}{3} x^{\frac{3}{2}}+2 x^{\frac{1}{2}}+C$

Hence, the correct Answer is C.

22. If $\frac{d}{d x} f(x)=4 x^{3}-\frac{3}{x^{4}}$ such that $f(2)=0$. Then $f(x)$ is

(A) $x^{4}+\frac{1}{x^{3}}-\frac{129}{8}$

(B) $x^{3}+\frac{1}{x^{4}}+\frac{129}{8}$

(C) $x^{4}+\frac{1}{x^{3}}+\frac{129}{8}$

(D) $x^{3}+\frac{1}{x^{4}}-\frac{129}{8}$

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Solution

It is given that,

$\frac{d}{d x} f(x)=4 x^{3}-\frac{3}{x^{4}}$

$\therefore$ Anti derivative of $4 x^{3}-\frac{3}{x^{4}}=f(x)$ $\therefore f(x)=\int 4 x^{3}-\frac{3}{x^{4}} d x$

$f(x)=4 \int x^{3} d x-3 \int(x^{-4}) d x$

$f(x)=4(\frac{x^{4}}{4})-3(\frac{x^{-3}}{-3})+C$

$\therefore(x)=x^{4}+\frac{1}{x^{3}}+C$

Also,

$f(2)=0$

$\therefore f(2)=(2)^{4}+\frac{1}{(2)^{3}}+C=0$

$\Rightarrow 16+\frac{1}{8}+C=0$

$\Rightarrow C=-(16+\frac{1}{8})$

$\Rightarrow C=\frac{-129}{8}$

$\therefore f(x)=x^{4}+\frac{1}{x^{3}}-\frac{129}{8}$

Hence, the correct Answer is A.



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