+0  
 
+5
299
7
avatar+16 

$$$\int \frac{1}{\left(x^2+\frac{3}{4}\right)}dx$$$

 

Somobody who know how to solve this?? Please :)

blaster01  Dec 7, 2014

Best Answer 

 #6
avatar+78618 
+10

I did this one a little differently...there is a "fudge factor" involved, but it's only a slight "fudge"

∫ x / √(3 - 4x^2) dx

let u = 3 - 4x^2    du  = -8x dx    du/-8 = x dx

So we have

-(1/8)∫u^(-1/2) du =

-(1/4)u^(1/2) + C =

-(1/4)(3 - 4x^2)^(1/2) + C

 

 

 

-

CPhill  Dec 8, 2014
Sort: 

7+0 Answers

 #1
avatar+26328 
+10

Integral:

 

.

Alan  Dec 7, 2014
 #2
avatar+91001 
+5

(2/sqrt3)tan^(-1)(2x/sqrt3)+c

 

$$\int\;\frac{1}{x^2+\frac{3}{4}}\;dx\;=\;\frac{2}{\sqrt3}\;tan^{-1}\;\frac{2x}{\sqrt3}+c$$

 

$$\int\;\frac{1}{x^2+a^2}\;dx\;=\;\frac{1}{a}\;tan^{-1}\;\frac{x}{a}+c$$

Melody  Dec 7, 2014
 #3
avatar+16 
0

thank you alan a lot, but i have one more problem with this:

 

$$$\int \left(\frac{x}{\sqrt{3-4x^2}}\right)dx$$$

 

can you help me please??

blaster01  Dec 7, 2014
 #4
avatar+91001 
+10

Some careless errors have been fixed - Thanks very much Alan.

It is correct now.

I have been perfecting my trademark here.

 If there is a R E A L L Y  L O N G  W A Y to do something I WILL FIND IT.

In my opinion CPhill's answer is the best one here.

It is really elegant.    Thanks Chris.

Heureka's answer is also much better than mine.    Thanks Heureka   

 

 

$$\int \left(\frac{x}{\sqrt{3-4x^2}}\right)dx\\\\
=\int \left(\frac{x}{\sqrt{4*(\frac{3}{4}-x^2)}}\right)dx\\\\
=\int \frac{x}{2\sqrt{(\frac{3}{4}-x^2)}}\;dx\\\\
=\frac{1}{2}\;\int \frac{x}{\sqrt{(\frac{3}{4}-x^2)}}\;dx\\\\
=\frac{1}{2}\;\int \frac{x}{\sqrt{(\frac{\sqrt{3}}{2})^2-x^2}}\;dx\\\\$$

 

$$\\=\frac{1}{2}\;\int \frac{x}{\sqrt{a^2-x^2}}\;dx\qquad where \quad a=\frac{\sqrt3}{2}\\\\
=\frac{1}{2}\;\int\;vu' \;dx\qquad where \quad v=x\;\;and\;\;u'=\frac{1}{\sqrt{a^2-x^2}}\\\\$$

 

 Now use integration by parts to solve.

 

 

$$\\v=x\;\;and\;\;u'=\frac{1}{\sqrt{a^2-x^2}}\\\\
v'=1\qquad u=sin^{-1}\;\frac{x}{a}\\\\
\;\int\; x*\frac{1}{\sqrt{a^2-x^2}}\;dx\\\\$$

 

$$\\=\frac{1}{2}(sin^{-1}\;\frac{x}{a}\;*\;x\;\;-\;\;\int\;sin^{-1}\;\frac{x}{a}\;*\;1\;dx)\\\\
=\frac{1}{2}\left(xsin^{-1}\;\frac{x}{a}\;\;-\left[a\sqrt{1-\frac{x^2}{a^2}}\;+\;xsin^{-1}\;\frac{x}{a}\;\;\right]\right)+c\\\\
=\frac{1}{2}\left(\;-\left[a\sqrt{1-\frac{x^2}{a^2}}\;\right]\right)+c\\\\
=\frac{-1}{2}\left(a\;\sqrt{1-\frac{x^2}{a^2}}\;\right)+c\\\\
=\frac{-1}{2}\left(\frac{\sqrt{3}}{2}\;\sqrt{1-\frac{4x^2}{3}}\;\right)+c\\\\
=\frac{-1}{2}\left(\frac{\sqrt{3}}{2}\;\sqrt{\frac{3-4x^2}{3}}\;\right)+c\\\\
=\frac{-1}{2}\left(\frac{1}{2}\;\sqrt{3-4x^2}\;\right)+c\\\\
=\;\frac{-\sqrt{3-4x^2}}{4}\;+c\\\\$$

Melody  Dec 8, 2014
 #5
avatar+18712 
+10

$$$\int \left(\frac{x}{\sqrt{3-4x^2}}\right)\ dx \quad \text { ?}$$$

$$\small{
\text{
$
\begin{array}{rcl}
&=&\int \left(\frac{ \big{x} }{\sqrt{3\left(1-\frac{4}{3}x^2\right)}} \right) \ dx
\\ \\
&=&\frac{1 }{\sqrt{3}}\int \left(\frac{ \big{x} }{\sqrt{ 1-
\left( \frac{ \big{x} }{ \sqrt \frac{3}{4} } \right)^2 } } \right) \ dx
\end{array}
$
}}
$\\\\$
\small\text{
we substitue: $ \frac{x} {\sqrt{ \frac{3}{4} } } = \sin(u) \quad \Rightarrow \quad \frac{ \ dx} {\sqrt{ \frac{3}{4} } } = \cos(u) \ du$
}}
$\\\\$
\small\text{
and set also: $ x = ( \sqrt{ \frac{3}{4} } ) * \sin(u) \quad $ and $\quad \ dx = ( \sqrt{ \frac{3}{4} } ) * \cos(u) \ du$
}}
$\\\\$
\small\text{
$
=\frac{1 }{\sqrt{3}}\int \left(\frac{ ( \big{ \sqrt{ \frac{3}{4} } ) * \sin(u) } }{\sqrt{ 1-
\big{ \left( \sin(u) \right)^2 } } } \right) ( \sqrt{ \frac{3}{4} } ) * \cos(u) \ du$
}}$$

$$$\\\\$
\small\text{
$
=\frac{1 }{\sqrt{3}}\int \left(
\frac{ ( \big{ \sqrt{
\frac{3}{4}
} ) * \sin(u)
}
}
{ \big{\cos(u) } }
\right) ( \sqrt{ \frac{3}{4} } ) * \cos(u) \ du$
}}
$\\\\$
\small\text{
$
=\dfrac{\frac{3}{4} }{ \sqrt{3} }\int \left(
\sin(u) \ du$
}}
$\\\\$
\small\text{
$
=\frac{ \sqrt{3}}{4}\int \left(
\sin(u) \ du$
}}
$\\\\$
\small\text{
$
=\frac{ \sqrt{3}}{4}\int \left(
\sin(u) \ du \quad | \quad \int\sin(u)\ du = -\cos(u)$
}}
$\\\\$
\small\text{
$
=\frac{ \sqrt{3}}{4}(-\cos(u))$
}}
$\\\\$
\small\text{
$
=-\frac{ \sqrt{3}}{4}\cos(u) \quad | \quad cos(u) = \sqrt{1-\sin(u)^2 }= \sqrt{1-\frac{4}{3}x^2 } $
}}
$\\\\$
\small\text{
$
=-\frac{ \sqrt{3}}{4}\sqrt{1-\frac{4}{3}x^2 } }$
}}
$\\\\$
\small\text{
$
=-\frac{1}{4}\sqrt{3-4x^2 } }$
}}$$

$$\boxed{\int \left(\frac{x}{\sqrt{3-4x^2}}\right)\ dx =-\frac{1}{4}\sqrt{3-4x^2 } \quad + c }$$

heureka  Dec 8, 2014
 #6
avatar+78618 
+10
Best Answer

I did this one a little differently...there is a "fudge factor" involved, but it's only a slight "fudge"

∫ x / √(3 - 4x^2) dx

let u = 3 - 4x^2    du  = -8x dx    du/-8 = x dx

So we have

-(1/8)∫u^(-1/2) du =

-(1/4)u^(1/2) + C =

-(1/4)(3 - 4x^2)^(1/2) + C

 

 

 

-

CPhill  Dec 8, 2014
 #7
avatar+91001 
0

Chris, I really like what you have done.  It is really neat and looks perfectly valid too me.  

 

Heureka's answer is also much better than mine.   Thanks Heureka    

Melody  Dec 9, 2014

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