200 days streak special 5

Calculus Level 4

0 2 d x ( 17 + 8 x 4 x 2 ) ( e 6 ( 1 x ) + 1 ) = ? \large \displaystyle \int_0^2 \dfrac{{dx}}{(17 + 8x - 4x^2)(e^{6(1-x)} + 1)} = \, ?

None of these 1 8 21 ln ( 21 + 2 21 2 ) \dfrac{1}{8\sqrt{21}} \ln \left(\dfrac{\sqrt{21} + 2}{\sqrt{21} - 2}\right) 1 21 ln ( 21 + 2 21 2 ) \dfrac{1}{\sqrt{21}} \ln \left(\dfrac{\sqrt{21} + 2}{\sqrt{21} - 2}\right) 1 2 21 ln ( 21 + 2 21 2 ) \dfrac{1}{2\sqrt{21}} \ln \left(\dfrac{\sqrt{21} + 2}{\sqrt{21} - 2}\right) 1 4 21 ln ( 21 + 2 21 2 ) \dfrac{1}{4\sqrt{21}} \ln \left(\dfrac{\sqrt{21} + 2}{\sqrt{21} - 2}\right)

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1 solution

Aditya Malusare
Jan 5, 2016

Let J = 0 2 d x ( 17 + 8 x 4 x 2 ) ( e 6 ( 1 x ) + 1 ) J = \displaystyle \int_0^2 \dfrac{{\text{d}x}}{(17 + 8x - 4x^2)(e^{6(1-x)} + 1)} be the required integral.

Using the property a b f ( t ) d t = a b f ( a + b t ) d t \displaystyle \int_a^b f(t) \text{d}t =\displaystyle \int_a^b f(a + b - t) \text{d}t , we get

2 J = 0 2 d x ( 17 + 8 x 4 x 2 ) ( e 6 ( 1 x ) + 1 ) + 0 2 d x ( 17 + 8 x 4 x 2 ) ( e 6 ( 1 x ) + 1 ) 2J = \int_0^2 \dfrac{{\text{d}x}}{(17 + 8x - 4x^2)(e^{6(1-x)} + 1)} + \int_0^2 \dfrac{{\text{d}x}}{(17 + 8x - 4x^2)(e^{-6(1-x)} + 1)} 2 J = 0 2 e 6 ( 1 x ) + 1 ( 17 + 8 x 4 x 2 ) ( e 6 ( 1 x ) + 1 ) d x = 0 2 d x 17 + 8 x 4 x 2 2J = \int_0^2 \dfrac{e^{6(1-x)} + 1 }{(17 + 8x - 4x^2)(e^{6(1-x)} + 1)} \text{d}x = \int_0^2 \dfrac{\text{d}x}{17 + 8x - 4x^2}

This integral can be written as 0 2 d x 21 4 ( x 1 ) 2 \displaystyle \int_0^2 \dfrac{\text{d}x}{21 - 4(x-1)^2} which can be evaluated using standard techniques to give J = 1 4 21 ln ( 21 + 2 21 2 ) J = \boxed{\dfrac{1}{4\sqrt{21}} \ln \left(\dfrac{\sqrt{21} + 2}{\sqrt{21} - 2}\right)}

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