Integration.

Calculus Level 3

d x ( x 3 ) 2 ( x 4 ) 2 = ( 1 1 x p ) + log ( 1 + 1 x r ) q ( 1 + 1 x r ) + C \large \int\frac{\,dx}{(x-3)^2(x-4)^2}=\left(1-\frac{1}{x-p}\right)+\log\left(1+\frac{1}{x-r}\right)^q-\left(1+\frac{1}{x-r}\right)+ C

Positive integers p p , q q and r r , where p p and r r are coprime integers and q q is a prime, satisfy the equation above. Find p + q + r p+q+r .

Notation: C C denotes the constant of integration .


The answer is 9.

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1 solution

I = d x ( x 3 ) 2 ( x 4 ) 2 By partial fractions decomposition – see below = ( 2 x 3 + 1 ( x 3 ) 2 2 x 4 + 1 ( x 4 ) 2 ) d x = 2 ln ( x 3 ) 1 x 3 2 ln ( x 4 ) 1 x 4 + C where C is the constant of integration. = 2 ln ( x 3 x 4 ) + 1 1 x 3 1 1 x 4 + C = ( 1 1 x 3 ) + ln ( x 4 + 1 x 4 ) 2 ( 1 + 1 x 4 ) + C = ( 1 1 x 3 ) + ln ( 1 + 1 x 4 ) 2 ( 1 + 1 x 4 ) + C \begin{aligned} I & = \int \frac {dx}{(x-3)^2(x-4)^2} & \small \color{#3D99F6} \text{By partial fractions decomposition -- see below} \\ & = \int \left( \frac 2{x-3} + \frac 1{(x-3)^2} - \frac 2{x-4} + \frac 1{(x-4)^2} \right) dx \\ & = 2 \ln (x-3) - \frac 1{x-3} - 2 \ln (x-4) - \frac 1{x-4} + \color{#3D99F6}C & \small \color{#3D99F6} \text{where }C \text{ is the constant of integration.} \\ & = 2 \ln \left(\frac {x-3}{x-4} \right) + 1 - \frac 1{x-3} - 1 - \frac 1{x-4} + C \\ & = \left(1 - \frac 1{x-3}\right) + \ln \left(\frac {x-4+1}{x-4} \right)^2 - \left(1 + \frac 1{x-4} \right) + C \\ & = \left(1 - \frac 1{x-3}\right) + \ln \left(1 + \frac 1{x-4} \right)^2 - \left(1 + \frac 1{x-4} \right) + C \end{aligned}

p + q + r = 3 + 2 + 4 = 9 \implies p+q+r = 3+2+4 = \boxed{9}


Note: Assuming that (reference: Partial Fractions - Repeated Factors )

Let 1 ( x 3 ) 2 ( x 4 ) 2 = A x 3 + B ( x 3 ) 2 + C x 4 + D ( x 4 ) 2 \small \begin{aligned} \frac 1{(x-3)^2(x-4)^2} & = \frac A{x-3} + \frac B{(x-3)^2} +\frac C{x-4} + \frac D{(x-4)^2} \end{aligned} . Multiplying both sides by ( x 3 ) 2 ( x 4 ) 2 \small (x-3)^2(x-4)^2 and flipping sides.

A ( x 3 ) ( x 4 ) 2 + B ( x 4 ) 2 + C ( x 3 ) 2 ( x 4 ) + D ( x 3 ) 2 = 1 Let x = 3 B = 1 Let x = 4 D = 1 Let x = 2 4 A + 4 B 2 C + D = 1 2 A + C = 2 . . . ( 1 ) Let x = 5 2 A + B + 4 C + 4 D = 1 2 A + 4 C = 4 . . . ( 2 ) ( 2 ) ( 1 ) C = 2 Substitute in ( 1 ) A = 2 \small \begin{aligned} A(x-3)(x-4)^2 + B(x-4)^2 + C(x-3)^2(x-4) + D(x-3)^2 & = 1 & \small \color{#3D99F6} \text{Let }x = 3 \\ \implies B & = 1 & \small \color{#3D99F6} \text{Let }x = 4 \\ \implies D & = 1 & \small \color{#3D99F6} \text{Let }x = 2 \\ -4A+4B-2C + D & = 1 \\ \implies 2A + C & = 2 \quad ...(1) & \small \color{#3D99F6} \text{Let }x = 5 \\ 2A + B + 4C + 4D & = 1 \\ \implies 2A + 4C & = - 4 \quad ...(2) & \small \color{#3D99F6} (2)-(1) \\ \implies C & = - 2 & \small \color{#3D99F6} \text{Substitute in }(1) \\ \implies A & = 2 \end{aligned}

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