To the power 11

Calculus Level 3

x 3 + x ( x 4 + 2 x 2 + 3 ) 11 d x \large \int \dfrac {x^3+x}{(x^4+2x^2+3)^{11}}\, dx

The indefinite integral above can be expressed as

( a x d + b x e + c ) f g + C , \frac {(ax^d+bx^e+c)^f}g + C,

where a , b , c , d , e , f , g a,b,c,d,e,f,g are all integer constants, where d > e d>e , and C C represents the arbitrary constant of integration .

Find a + b c + d e + f g a+b-c+d-e+f-g .


The answer is 32.

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2 solutions

Chew-Seong Cheong
May 22, 2017

I = x 3 + x ( x 4 + 2 x 2 + 3 ) 11 d x Let u = x 4 + 2 x 2 + 3 , d u = 4 x 3 + 4 x d x = d u 4 u 11 = 1 40 u 10 + C = 1 40 ( x 4 + 2 x 2 + 3 ) 10 + C \begin{aligned} I & = \int \frac {x^3+x}{(x^4+2x^2+3)^{11}} dx & \small \color{#3D99F6} \text {Let }u =x^4+2x^2+3, du = 4x^3+4x \ dx \\ &=\int \frac {du}{4u^{11}} \\ &= -\frac 1{40u^{10}}+C\\ &=-\frac 1{40(x^4+2x^2+3)^{10}} +C \end{aligned}

a + b c + d e + f g = 1 + 2 3 + 4 2 10 + 40 = 32 \implies a+b-c+d-e+f-g = 1+2-3+4-2-10+40=\boxed{32}

Ayon Ghosh
May 22, 2017

there may be better ways to do this but right now I now only of the u substitution way so I came up with this solution.

Let u = ( x + 1 ) u = (x+1)

notice ( x 4 + 2 x 2 + 3 ) 11 (x^4 + 2x^2 + 3)^{11} can be written as ( ( x + 1 ) 2 + 2 ) 11 ((x+1)^2 + 2)^{11} .

also x 3 + x x^3 +x can also be written as x ( x 2 + 1 ) x(x^2+1) .

also d u = d x du = dx so the integral can be written as u / ( u + 2 ) 11 1 / 2 d u u / (u+2)^{11} * 1/2 * du .we take the 1 / 2 1/2 out of the integral sign (constant).

now we again substitute u 2 = v u^2 = v . also d v = 2 u d u dv = 2u du .

again the indefinite integral becomes 1 / 4 1/4 (integral sign) ( v + 2 ) 11 d v (v+2)^{-11} dv which is easy to evaluate and we get

( v + 2 ) 10 / 10 (v+2)^{-10} / -10 + + C 1 C_1

now we back substitute u u and x x to get,

( x 4 + 2 x 2 + 3 ) 10 (x^4 + 2x^2 + 3)^{-10} / / 40 -40 + + C 1 C_1

compare co efficients and get the answer.

@Jaydee Lucero done :-) !

Ayon Ghosh - 3 years, 11 months ago

How does u = x + 1 u = x+1 imply d u = 2 x d x du = 2xdx ?

Ankit Kumar Jain - 4 years ago

@Ankit Kumar Jain sorry it is meant to be x dx I have modified it.

Ayon Ghosh - 4 years ago

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Shouldn't it be du = dx ?

Ankit Kumar Jain - 4 years ago

@Ankit Kumar Jain was in a hurry so did the double mistakes.

Ayon Ghosh - 4 years ago

@Ankit Kumar Jain just to make up for my mistakes I offer to you this nice maxima problem check it out https://brilliant.org/problems/maxi-mini-ma/

Ayon Ghosh - 4 years ago

@Ayon Ghosh just a suggestion: perhaps you can change the variable for arbitrary constant of integration to C 1 C_1 so as to avoid confusion with the C C inside the parentheses. :)

Jaydee Lucero - 3 years, 11 months ago

It's a simple question why make it long.

Praveen Kumar - 3 years, 1 month ago

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