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  • Please solve RD Sharma class 12 chapter Indefinite Integrals exercise 18.9 question 69 maths textbook solution

Please solve RD Sharma class 12 chapter Indefinite Integrals exercise 18.9 question 69 maths textbook solution

Answers (1)

Answer: -\frac{20}{3}\left(5-x^{2}\right)^{\frac{3}{2}}+\frac{4}{5}\left(5-x^{2}\right)^{\frac{5}{2}}+c

Hint: Use substitution method to solve this integral

Given: \int 4 x^{3} \sqrt{5-x^{2}} \; d x

Solution:

        \begin{aligned} &\text { Let } I=\int 4 x^{3} \sqrt{5-x^{2}} \; d x \\ &\text { Put } 5-x^{2}=t^{2} \Rightarrow-2 x \; d x=2 t\; d t \\ &\Rightarrow d x=\frac{t}{-x} d t \text { then } \end{aligned}

        I=\int 4 x^{3} \sqrt{t^{2}} \frac{t \cdot d t}{-x}=-\int 4 x^{2} \cdot t . t \; d t

            =-4 \int\left(5-t^{2}\right) t^{2} d t=-4 \int\left(5 t^{2}-t^{2} \cdot t^{2}\right) d t \quad\left[\because 5-x^{2}=t^{2} \Rightarrow x^{2}=5-t^{2}\right]

            =-4 \int\left(5 t^{2}-t^{4}\right) d t

            =-20 \int t^{2} d t+4 \int t^{4} d t

            =-20 \cdot \frac{t^{2+1}}{2+1}+4 \cdot \frac{t^{4+1}}{4+1}+\mathrm{c} \quad\left[\because \int x^{n} d x=\frac{x^{n+1}}{n+1}+c\right]

            =-20 \cdot \frac{t^{3}}{3}+4 \frac{t^{5}}{5}+c

            =-\frac{20}{3}\left(\sqrt{5-x^{2}}\right)^{3}+\frac{4}{5}\left(\sqrt{5-x^{2}}\right)^{5}+c \quad\left[\because t^{2}=5-x^{2} \Rightarrow t=\sqrt{5-x^{2}}\right]

            =-\frac{20}{3}\left(5-x^{2}\right)^{\frac{3}{2}}+\frac{4}{5}\left(5-x^{2}\right)^{\frac{5}{2}}+c

 

 

        

 

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