Question
Choose the correct answer in Exercise:

$\int^\frac{2}{3}_{0}\frac{\text{dx}}{4+9\text{x}^{2}}\text{equals}$

  1. $\frac{\pi}{6}$

  2. $\frac{\pi}{12}$

  3. $\frac{\pi}{24}$

  4. $\frac{\pi}{4}$

Answer

$\frac{\pi}{24}$

$\int\frac{\text{dx}}{4+9\text{x}^{2}}=\int\frac{\text{dx}}{(2)^{2}+(3\text{x})^{2}}$

$\ \text{put}\ 3\text{x}=\text{t}\Rightarrow3\text{dx}=\text{dt}$

$\therefore\int\frac{\text{dx}}{(2)^{2}+(3\text{x})^{2}}=\frac{1}{3}\int\frac{\text{dt}}{(2)^{2}+\text{t}^{2}}$

$=\frac{1}{3}\bigg[\frac{1}{2}\tan^{-1}\frac{t}{2}\bigg]$

$=\frac{1}{6}\tan^{-1}\bigg(\frac{3\text{x}}{2}\bigg)$

$=\text{F}\text{(x)}$

By second fundamental theorem of calculus, we obtain

$\int\limits_{0}^{\frac{2}{3}}\frac{\text{dx}}{4+9\text{x}^{2}}=\text{F}\bigg(\frac{2}{3}\bigg)-\text{F}(0)$

$=\frac{1}{6}\tan^{-1}\bigg(\frac{3}{2}.\frac{2}{3}\bigg)-\frac{1}{6}\tan^{-1}0$

$=\frac{1}{6}\tan^{-1}1-0$

$=\frac{1}{6}\times\frac{\pi}{4}$

$=\frac{\pi}{24}$

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