'An' Integral

Calculus Level 4

If ( x 1 ) 3 x d x = a b x 7 2 d e x 5 2 + u x 3 2 + v x 1 2 + C \large \int_{}^{} \frac{(x-1)^3}{\sqrt x} dx = \frac{a}{b}x^{\frac{7}{2}}- \frac{d}{e}x^{\frac{5}{2}}+ ux^{\frac{3}{2}}+ vx^{\frac{1}{2}}+C

where C C is any constant and the pairs ( a , b ) (a,b) and ( d , e ) (d,e) are pairs of co-prime positive integers. Then find the value a + b + d + e + u + v a+b+d+e+u+v

Clarification: -Rational no.s a b \frac{a}{b} and d e \frac{d}{e} are in the standard form.


The answer is 20.

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2 solutions

Romain Bouchard
Feb 12, 2018

( x 1 ) 3 x d x = ( x 1 ) 3 x d x = x 3 3 x 2 + 3 x 1 { x 1 / 2 d x \int \frac{(x-1)^3}{\sqrt{x}}dx = \int \frac{(x-1)^3}{\sqrt{x}}dx = \int \frac{x^3-3x^2+3x-1}{\{x^{1/2}}dx

( x 1 ) 3 x d x = ( x 5 / 2 3 x 3 / 2 + 3 x 1 / 2 x 1 / 2 ) d x = 2 7 x 7 / 2 6 5 x 5 / 2 + 2 x 3 / 2 2 x 1 / 2 \Rightarrow \int \frac{(x-1)^3}{\sqrt{x}}dx = \int (x^{5/2}-3x^{3/2}+3x^{1/2}-x^{-1/2})dx = \frac{2}{7}x^{7/2}-\frac{6}{5}x^{5/2}+2x^{3/2}-2x^{1/2}

a = 2 , b = 7 , d = 6 , e = 5 , u = 2 , v = 2 a + b + d + e + u + v = 2 + 7 + 6 + 5 + 2 2 = 20 \Rightarrow a=2, b=7, d=6, e=5, u=2, v=-2 \Rightarrow a+b+d+e+u+v = 2+7+6+5+2-2 = 20

Chew-Seong Cheong
Feb 13, 2018

I = ( x 1 ) 3 x d x Let u 2 = x 2 u d u = d x = ( u 2 1 ) 3 u 2 u d u = 2 ( u 6 3 u 4 + 3 u 2 1 ) d u = 2 7 u 7 6 5 u 5 + 2 u 3 2 u + C where C is the constant of integration. = 2 7 x 7 2 6 5 x 5 2 + 2 x 3 2 2 x 1 2 + C Putting back u = x \begin{aligned} I & = \int \frac {(x-1)^3}{\sqrt x}dx & \small \color{#3D99F6} \text{Let }u^2 = x \implies 2u\ du = dx \\ & = \int \frac {(u^2-1)^3}u \cdot 2u\ du \\ & = 2 \int \left(u^6-3u^4+3u^2-1\right)\ du \\ & = \frac 27u^7 - \frac 65 u^5 + 2u^3 - 2u + \color{#3D99F6}C & \small \color{#3D99F6} \text{where } C \text{ is the constant of integration.} \\ & = \frac 27x^\frac 72 - \frac 65 x^\frac 52 + 2x^\frac 32 - 2x^\frac 12 + C & \small \color{#3D99F6} \text{Putting back } u = \sqrt x \end{aligned}

Therefore, a + b + d + e + u + v = 2 + 7 + 6 + 5 + 2 2 = 20 a+b+d+e+u+v = 2+7+6+5+2-2 = \boxed{20} .

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