Ω = 3 3 ( lo g ϕ + n = 1 ∑ ∞ ( 4 n − 1 ) ! ( ( 2 n − 1 ) ! ) 2 ) − 4 ∫ 0 1 1 − x 2 d x
The value of Ω defined above can be expressed as a a ( lo g ϕ + c d b ln ( c e − a d ) ) where the unknowns are positive integer and a , c , d , e are primes. Find the value of a + b + c + d + e .
Source : This problem is taken from Romanian Mathematical Magazine and proposed by Prof. Daniel Sitaru .
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My approach
Given integral I = 4 ∫ 0 1 1 − x 2 d x represent the total area of circle of radius r = 1 . so I = π . Also the integral can be solved by beta function.
Evaluting the sum as n = 1 ∑ ∞ ( 4 n − 1 ) ! ( ( 2 n − 1 ) ! ) 2 = n = 1 ∑ ∞ ∫ 0 1 t 2 n − 1 ( 1 − t ) 2 n − 1 d t = ∫ 0 1 ( t 2 − t − 1 ) ( t 2 − t + 1 ) t 2 − t d t = 2 1 ∫ 0 1 ( t 2 − t − 1 1 + t 2 − t + 1 1 ) d t = 2 1 ∫ 0 1 ( ( t − 2 1 ) 2 − 4 5 1 + ( t − 2 1 ) 2 + 4 3 1 ) d t By the use of standard integration formula we have = 5 2 tan − 1 ( 3 2 t − 1 ) + 2 3 1 ln [ ∣ 2 t − 1 + 5 ∣ ∣ 2 t − 1 − 5 ∣ ] now setting then limits we deduce = 3 3 π + 2 5 1 ln [ 3 + 5 3 − 5 ] Thus Ω = 3 3 ( lo g ϕ + 3 3 π + 2 5 1 ln [ 3 + 5 3 − 5 ] ) − π = 3 3 lo g ϕ + 2 5 3 3 ln [ 3 + 5 3 − 5 ] = 3 3 ( lo g ϕ + 2 5 1 ln [ 2 7 − 3 5 ] ) gives us a + b + c + d + e = 1 8