Evaluate the integral submit your answer as .
This problem, taken from Romanian Mathematical Magazine , proposed by Sir Srinivasa Raghava .
The original proposed problem supposed to evaluate the integral is closed form.
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One of the approach which I wish to share. ∫ 0 1 x + 3 x ( x − 3 x ) ln ( x + 3 x ) d x = x = u 3 3 ∫ 0 1 u 2 + 1 u ( u 3 − u ) ln ( u 3 + u ) d u the latter integral we write as 3 − 1 I = ∫ 0 1 ( u 2 + 1 u 4 ln ( u 3 + u ) − u 2 + 1 u 2 ln ( u 3 + u ) ) let u = tan y then the integral becomes ∫ 0 4 π tan 4 y ln ( tan y sec 2 y ) − tan 2 y ln ( tan y sec 2 y ) d y I 1 ∫ 0 4 π ( tan 4 y − tan 2 y ) ln ( tan y ) d y + I 2 ∫ 0 4 π ( tan 4 y − tan 2 y ) ln ( sec 2 y ) d y By using the reduction formula of I ( n ) = ∫ tan n y d y for n = 4 we deduce that I ( 4 ) = ∫ tan 4 y = y + 3 1 sec 2 y tan y − 3 4 + C and n = 2 is trivial to see I ( 2 ) = ∫ tan 2 y = tan y − y + C Now we calculate I 1 , I 2 using integration by parts I 1 = 0 ln ( tan y ) ∫ 0 4 π ( tan 4 y − tan 2 y ) d y − ∫ 0 4 π tan y sec 2 y ( I ( 4 ) − I ( 2 ) ) d y Further I 1 = ∫ 0 4 π tan y sec 2 y ( 2 y + 3 tan y ( sec 2 y − 7 ) ) d y = I 3 ∫ 0 4 π 2 ( y tan y + y cot y ) d y + I 4 3 1 ∫ 0 4 π sec 2 y ( sec 2 y − 7 ) d y Here I 3 = y ∫ 0 4 π ( tan y + cot y ) 0 − ∫ 0 4 π ( ∫ ( tan y + cot y ) d y ) d y = − ∫ 0 4 π ln ( tan y ) d y = tan y = t − ∫ 0 1 1 + t 2 ln t d t = − n = 1 ∑ ∞ ( 2 n − 1 ) 2 ( − 1 ) n = G ⇒ I 3 = − 2 G Further we evaluate 3 I 4 = ∫ 0 4 π sec 2 y ( sec 2 y − 7 ) d y = ∫ 0 4 π ( sec 2 y tan 2 y − 6 tan 2 y ) = [ 3 tan 3 y − 6 tan y ] 0 4 π = − 3 1 7 ⇒ I 4 = 9 1 7 and hence I 1 = ∫ 0 4 π ( tan 4 y − tan 2 y ) ln ( tan y ) d y = 9 1 7 − 2 G Now we evaluate I 2 ∫ 0 4 π ( tan 4 y − tan 2 y ) ln ( sec 2 y ) d y = − 2 ∫ 0 4 π ( tan 4 y − tan 2 y ) ln ( cos y ) d y = I B P − 2 ln ( cos y ) ∫ 0 4 π ( tan 4 − tan 2 y ) d y + 2 ∫ 0 4 π ( ( − tan y ) ∫ ( tan 4 y − tan 2 y ) d y ) d y = − 3 5 ln 2 + 2 π ln 2 − 2 ∫ 0 4 π tan y ( 2 y + 3 1 sec 2 y tan y − 3 7 tan y ) d y Further ∫ 0 4 π ( − 4 y tan y − 3 2 sec 2 y tan 2 y + 3 1 4 tan 2 y ) d y = − 4 ∫ 0 4 π y tan y + 9 4 0 − 6 7 π
= − 4 y ∫ 0 4 π tan y + 4 ∫ 0 4 π ( ∫ tan y d y ) d y = − 2 π ln 2 − 4 ∫ 0 4 π ln ( cos y ) d y Let J 1 = ∫ 0 4 π ln ( cos y ) d y , J 2 = ∫ 0 4 π ln ( sin y ) d y Note that J 1 + J 2 = ∫ 0 4 π ln ( 2 1 sin 2 y ) d y = 2 y = t 2 1 ∫ 0 2 π ln ( sin t ) d t − 4 π ln 2 = − 4 π ln 2 − 4 π ln 2 = − 2 π ln 2 Also J 2 − J 1 = ∫ 0 4 π ln ( tan y ) d y = I 1 − G So = − 2 π ln 2 − 4 ∫ 0 4 π ln ( cos y ) d y = − 2 π ln 2 − 4 ( 2 G − 2 π ln 2 ) = 2 π ln 2 − 2 G Making the finally answer 3 ( I 1 + I 2 ) = 3 ( 9 4 0 + 1 7 − 4 G + π ln 2 − 3 5 ln 2 − 3 7 π ) = 1 9 − 1 2 G + 3 π ln 2 − 5 ln ( 2 ) − 2 7 π ≈ 0 . 0 7 9 8 6 0 9 5 0 4 2 ⋯
The solution I had is quite lengthy and to speed up the calculation and reduce the work I even tried up with generating function for harmonic numbers which seems to be tedious again with calculations. So im looking for any short solution/s. I will be glad to know if anyone wish to share their approach.