Crazy Integral

Calculus Level 3

Find

0 . 5 x 4 2 ( 1 x ) 5 + 4 x x 2 + 5 x + 6 d x \int _{ 0 }^{ .5 }{ \cfrac { x^{ 4 } }{ 2(1-x)^{ 5 } } } +\frac { 4x }{ x^{ 2 }+5x+6 } dx


The answer is 0.119567.

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1 solution

Niven Achenjang
Mar 22, 2014

Integrating this expression takes quite a few steps.

x 4 2 ( 1 x ) 5 + 4 x x 2 + 5 x + 6 d x \int { \cfrac { x^{ 4 } }{ 2(1-x)^{ 5 } } } +\frac { 4x }{ x^{ 2 }+5x+6 } dx

By setting x = s i n 2 ( u ) x= sin^{ 2 }(u) in the first fraction and breaking the second one apart using partial fractions, you get the following:

t a n 9 ( u ) d u + 8 x + 2 + 12 x + 3 d x \int { tan^{ 9 }(u)du } +\int { \frac { -8 }{ x+2 } +\frac { 12 }{ x+3 } dx }

The right integral here comes out as 8 l n x + 2 + 12 l n x + 3 -8ln|x+2| +12ln|x+3| so for now we will omit this from the integration and focus on the left integral. By setting y = t a n ( u ) y = tan(u) you get this integral

y 9 y 2 + 1 d y \int { \frac { y^{ 9 } }{ y^{ 2 }+1 } } dy

By dividing that out you arrive at

y 7 y 5 + y 3 y + y y 2 + 1 d y \int { y^{ 7 }-y^{ 5 }+y^{ 3 }-y+\frac { y }{ y^{ 2 }+1 } } dy

Most of it can be integrated rather easily, but the last part we'll inetegrate by setting y = t a n ( w ) y=tan(w) which gives us(I'm omitting the other terms because integration of them is simple)

t a n ( w ) d w = l n c o s ( w ) \int { tan(w)dw\quad =\quad -ln|cos(w)| }

When we put this all together, we get this final result

y = t a n ( a s i n ( x ) ) w = a s i n ( x ) y\quad =\quad tan(asin(\sqrt { x } ))\\ w\quad =\quad asin(\sqrt { x } )

y 8 8 y 6 6 + y 4 4 y 2 2 l n c o s ( w ) 8 l n x + 2 + 12 l n x + 3 \frac { y^{ 8 } }{ 8 } -\frac { y^{ 6 } }{ 6 } +\frac { y^{ 4 } }{ 4 } -\frac { y^{ 2 } }{ 2 } -ln|cos(w)|-8ln|x+2|+12ln|x+3|

After getting that result, you just have to plug and chug to get the correct answer ( Personally, I used desmos graphing calculator: https://www.desmos.com/calculator/ckyrrlf7dg )

Break the integral into 2 portions. Use the technique of substitution for the first portion: Let k = 1 - x dk/dx = -1 --> dx= -dk. Don't forget to modify the limits! Represent the terms of the first portion in terms of k. You would obtain several terms containing reciprocal powers of k.

Factorise the denominator of the second portion. One should obtain {x^2} + 5x +6 = (x + 3)*(x + 2) Use partial fractions for the second portion. Integrate, and you would obtain natural logarithms containing x.

Chester Gan - 7 years, 2 months ago

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That's way simpler than what I did. Good solution, man.

Niven Achenjang - 7 years, 2 months ago

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