>>288

dx/x(n+1-x) = rdt

{1/x + 1/(n+1-x)}(1/(n+1)) dx = rdt
log|x| -log|n+1-x| = rt +C
log(|x|/|n+1-x|) =rt+C
x/(n+1-x)=C'・exp(rt)
あと適当に変形