Question
Evaluate the following integrals:

$\int\frac{1}{\sqrt{7-6\text{x}-\text{x}^2}}\text{ dx}$

Answer

7 - 6x - x2 can be written as 7 - (x2 + 6x + 9 - 9).

Therefore,

$7-(\text{x}^2+6\text{x}+9-9)$

$=16-(\text{x}^2+6\text{x}+9)$

$=16-(\text{x}+3)^2$

$=(4)^2-(\text{x}+3)^2$

$\therefore\ \int\frac{1}{\sqrt{7-6\text{x}-\text{x}^2}}\text{ dx}$

$=\int\frac{1}{\sqrt{(4)^2-(\text{x}+3)^2}}\text{ dx}$

Let x + 3 = t

$\Rightarrow\text{dx}=\text{dt}$

$\Rightarrow\int\frac{1}{\sqrt{(4)^2-(\text{x}+3)^2}}\text{ dx}=\int\frac{1}{\sqrt{(4)^2-(\text{t})^2}}\text{ dt}$

$=\sin^{-1}\Big(\frac{\text{t}}{4}\Big)+\text{C}$

$=\sin^{-1}\Big(\frac{\text{x}+3}{4}\Big)+\text{C}$

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