2/(4n+1)= 1/(2n+1) +1/{(2n+1)(4n+1)} だから
1/(4n-1) - 1/(4n+1) = 1/{(2n+1)(4n-1)} +1/{(2n+1)(4n-1)(4n+1)}、
1/(4n+1) - 1/(4n+3) = 1/{(2n+1)(4n+3)} +1/{(2n+1)(4n+1)(4n+3)}