∫ 0 1 ∫ 0 1 1 + 2 + 2 + 2 + ⋯ x y x y x y d y d x
If the answer can be represented in the form b a ⎣ ⎢ ⎢ ⎡ − b a + b c 2 + ln 2 ( 1 + 2 a ) a ⎦ ⎥ ⎥ ⎤ , where a , b , c ∈ Z + and b a is in simplest form, find a + b + c .
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The integrand is F ( x , y ) = 1 + 1 + F ( x , y ) x y = 1 + x y and so the integral is I = ∫ 0 1 ∫ 0 1 F ( x , y ) d x d y = ∫ 0 1 d u ∫ u 1 v 1 + u d v = − ∫ 0 1 ln u 1 + u d u = 3 2 ∫ 0 1 u ( 1 + u ) 2 3 − 1 d u using first the change of variables u = x y , v = x , and then integrating by parts. Now the substitution z = 1 + u gives I = 3 2 ∫ 1 2 z 2 − 1 z 3 − 1 × 2 z d z = 3 4 ∫ 1 2 z 2 − 1 z 4 − z d z = 3 4 ∫ 1 2 ( z 2 + 1 − z + 1 1 ) d z = 3 4 [ 3 1 z 2 + z − ln ( z + 1 ) ] 1 2 = 3 4 [ − 3 4 + 3 5 2 + ln 2 − ln ( 1 + 2 ) ] = 3 4 [ − 3 4 + 3 5 2 + ln ( 2 ( 1 + 2 4 ) 4 ) ] making the answer 4 + 3 + 5 = 1 2 .