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 #3
avatar+2499 
+5
Feb 15, 2016
 #3
avatar+26403 
+10

Integrate: dx/sqrt[1 - x^2], for all x from -1 to 1.

 

\(\small{ \begin{array}{lrcll} &\int \limits_{-1}^{1} { \frac{1}{ \sqrt{1-x^2} } \ dx } \\ \hline \text{Substitution : } & x &=& \sin{(u)} \qquad \rightarrow \qquad u = \arcsin{(x)}\\ & \ dx &=& \cos{(u)}\ du \\ \hline &\int \limits_{-1}^{1} { \frac{1}{ \sqrt{1-x^2} } \ dx } &=&\int \limits_{-1}^{1} { \frac{1}{ \sqrt{1-\sin^2{(u)}} } \ dx } \\ & &=&\int \limits_{-1}^{1} { \frac{1}{ \sqrt{1-\sin^2{(u)}} } \ dx } \qquad | \qquad 1-\sin^2{(u)} = \cos^2{(u)}\\ & &=&\int \limits_{-1}^{1} { \frac{1}{ \sqrt{\cos^2{(u)}} } \ dx } \\ & &=&\int \limits_{-1}^{1} { \frac{1}{ \cos{(u)} } \ dx } \\ & &=&\int \limits_{-1}^{1} { \frac{1}{ \cos{(u)} } \ dx } \qquad | \qquad dx = \cos{(u)}\ du\\ & &=&\int \limits_{-1}^{1} { \frac{1}{ \cos{(u)} }\cdot \cos{(u)}\ du } \\ & &=&\int \limits_{-1}^{1} { \ du } \\ & &=& [ u ]_{-1}^{1} \qquad | \qquad u = \arcsin{(x)} \\ & \mathbf{ \int \limits_{-1}^{1} { \frac{1}{ \sqrt{1-x^2} } \ dx } } & \mathbf{=} & \mathbf{ [~ \arcsin{(x)} ~]_{-1}^{1} }\\ \hline & \int \limits_{-1}^{1} { \frac{1}{ \sqrt{1-x^2} } \ dx } & = & [~ \arcsin{(x)} ~]_{-1}^{1} \\ & & = & [~ \arcsin{(1)}-\arcsin{(-1)} ~]\\ & & = & [~ \frac{\pi}{2} -\frac{-\pi}{2} ~]\\ & & = & [~ \frac{\pi}{2} +\frac{\pi}{2} ~]\\ & & = & \pi \\ & \mathbf{ \int \limits_{-1}^{1} { \frac{1}{ \sqrt{1-x^2} } \ dx } } &\mathbf{=}& \mathbf{ \pi } \\ & \mathbf{ \int \limits_{-1}^{1} { \frac{1}{ \sqrt{1-x^2} } \ dx } } &\mathbf{=}& \mathbf{ 3.14159265359 } \end{array} }\)

laugh

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Feb 15, 2016
 #1
avatar+118733 
+5
Feb 15, 2016

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