STEP II 1997 question 3 solution - The Student Room
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STEP II 1997 question 3 solution

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\displaystyle \frac{ax+b}{x^2+2x+2} + \frac{cx+d}{x^2-2x+2} = \frac{1}{x^4+4}

Therefore

\displaystyle (ax+b)(x^2-2x+2) + (cx+d)(x^2+2x+2) = 1

Expand out

\displaystyle ax^3 -2ax^2 + 2ax + bx^2 -2bx + 2b + cx^3 +2cx^2 + 2cx + dx^2 + 2dx + 2d = 1

\displaystyle (a+c)x^3 (-2a+b+2c+d)x^2 + (2a-2b+2c+2d)x + 2b + 2d = 1

Comparing coefficients

 a + c = 0 - 1

 -2a+b+2c+d = 0 - 2

 2a-2b+2c+2d = 0 - 3

 2b+2d = 1 - 4

 a = -c - Substituting this into 2 and 3

 b + 4c + d = 0

 b = d - Substituting this into 4

 4b = 1

 b = \frac{1}{4}

 4d = 1

 d = \frac{1}{4}

Substituting values of b and d into 2

 \frac{1}{2} + 4c = 0

 c = -\frac{1}{8} - Substituting back into 1

 a = \frac{1}{8}

So

 a = \frac{1}{8}

 b = \frac{1}{4}

 c = -\frac{1}{8}

 d = \frac{1}{4}

Therefore

\displaystyle \frac{1}{x^4+4} = \frac{\frac{1}{8}x+\frac{1}{4}}{x^2+2x+2} + \frac{\frac{1}{4}-\frac{1}{8}x}{x^2-2x+2}

\displaystyle = \frac{x+2}{8x^2+16x+16} + \frac{2-x}{8x^2-16x+16}

 \displaystyle \int \frac{1}{x^4+4} = \int \frac{x+2}{8x^2+16x+16} + \frac{2-x}{8x^2-16x+16}

\displaystyle = \frac{1}{8}\int \frac{x+2}{x^2+2x+2} + \frac{2-x}{x^2-2x+2}

Consider \displaystyle \int \frac{x+2}{x^2+2x+2}

\displaystyle \int \frac{x+2}{x^2+2x+2} = \int \frac{2x+2}{x^2+2x+2} - \frac{x}{x^2+2x+2}

\displaystyle = \ln(x^2+2x+2) - \int \frac{x}{x^2+2x+2}

Consider  \displaystyle \int \frac{x}{x^2+2x+2}

 \displaystyle \int \frac{x}{x^2+2x+2} = \int \frac{2x+2}{x^2+2x+2} - \frac{2}{x^2+2x+2} - \frac{x}{x^2+2x+2}

 \displaystyle 2\int \frac{x}{x^2+2x+2} = ln (x^2+2x+2) - \int \frac{2}{x^2+2x+2}

\displaystyle = ln (x^2+2x+2) - \int \frac{2}{(x+1)^2+1}

let u = x+1

\displaystyle \frac{du}{dx} = 1

 du = dx

\displaystyle = \ln (x^2+2x+2) - 2\int\frac{1}{u^2+1}

\displaystyle = \ln(x^2+2x+2) - 2\arctanu

\displaystyle = \ln(x^2+2x+2) - 2\arctan(x+1)


 \displaystyle 2\int \frac{x}{x^2+2x+2} = \ln(x^2+2x+2) - 2\arctan(x+1)

\displaystyle \int \frac{x}{x^2+2x+2} = \frac{1}{2}\ln(x^2+2x+2) - \arctan(x+1)

Therefore

\displaystyle \int \frac{x+2}{x^2+2x+2} = \ln(x^2+2x+2) - \left[\frac{1}{2}ln(x^2+2x+2) - arctan(x+1)\right]

 = \frac{1}{2}ln(x^2+2x+2) + \arctan(x+1)

Consider  \displaystyle \int \frac{2-x}{x^2-2x+2}

 \displaystyle \int \frac{2-x}{x^2-2x+2} = \int \frac{2x-2}{x^2-2x+2} +\int \frac{4}{x^2-2x+2} - \int \frac{3x}{x^2-2x+2}

\displaystyle = \ln (x^2-2x+2) + 4\int \frac{1}{(x-1)^2+1} - 3 \int \frac{x}{x^2-2x+2}

Consider \displaystyle \int \frac{1}{(x-1)^2+1}

 u = x-1

 du = dx

\displaystyle \int \frac{1}{(x-1)^2+1} = \int \frac{1}{u^2+1}

 = \arctan(x-1)

 \displaystyle \int \frac{2-x}{x^2-2x+2} = \ln(x^2-2x+2) +4\arctan(x-1) - \int \frac{3x}{x^2-2x+2}

Consider \displaystyle \int \frac{3x}{x^2-2x+2}

\displaystyle \int \frac{3x}{x^2-2x+2} = \int \frac{2x-2}{x^2-2x+2} + \int \frac{x}{x^2-2x+2} + \int \frac{2}{x^2-2x+1}

 2\displaystyle \int \frac{x}{x^2-2x+2} = \ln (x^2-2x+2) + 2\arctan(x-1)

 \displaystyle \int \frac{x}{x^2-2x+2} = \frac{1}{2}\ln (x^2-2x+2) + \arctan(x-1)

Therefore

 \displaystyle \int \frac{2-x}{x^2-2x+2} = \ln(x^2-2x+2) +4\arctan(x-1) -3\left[\frac{1}{2}ln (x^2-2x+2) + arctan(x-1)\right]

Finally

 \displaystyle \int \frac{1}{x^4+4} = \frac{1}{8} \int \frac{x+2}{x^2+2x+2} +  \frac{2-x}{x^2-2x+2}

Unparseable or potentially dangerous latex formula. Error 5: Image dimensions are out of bounds: 1236x53
\displaystyle = \left[\frac{1}{8}\ln(x^2+2x+2) -\frac{1}{8}\left(\frac{1}{2}ln(x^2+2x+2)-arctan(x+1)\right) + \frac{1}{8}\ln(x^2-2x+2) + \frac{1}{2}\arctan(x-1) - \frac{3}{8}\left(\frac{1}{2}\ln(x^2-2x+2) + arctan(x-1)\right)\right]_0^1
Unparseable or potentially dangerous latex formula. Error 5: Image dimensions are out of bounds: 1396x48
\displaystyle = \left[\frac{1}{8}\ln5 - \frac{1}{8}\left(\frac{1}{2}\ln5 - \arctan2\right)+ \frac{1}{8}\ln1 + \frac{1}{2}\ln0 - \frac{3}{8}\left(\frac{1}{2}\ln1+\arctan0\right)\right] - \left[\frac{1}{8}\ln2 - \frac{1}{8}\left(\frac{1}{2}\ln2 - \arctan(1)\right)+\frac{1}{8}\ln2 + \frac{1}{2}\arctan(-1) -\frac{3}{8}\left(\frac{1}{2}\ln2+\arctan(-1)\right)\right]

\displaystyle =  \left[\frac{1}{16}\ln5 +\frac{1}{8}\arctan(2)\right] - \left[\frac{1}{16}\ln2 + \frac{\pi}{32} + \frac{1}{8}\ln2 -\frac{\pi}{8} - \frac{3}{16}\ln2 +\frac{3\pi}{32}\right]

\displaystyle =  \frac{1}{16}\ln5 + \frac{1}{8}\arctan2

Solution by insparato

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