I was trying to prove some of the results related to beta function and I came across this step
Therefore, $$\frac{B(m+1,n)}{m}=\frac{B(m,n+1)}{n}=k,\quad\mathrm{say}\quad\ldots(i)$$ This yields $$k=\frac{B(m+1,n)+B(m,n+1)}{m+n}$$
Can anyone tell me how they got the value of $k$? Thanks in advance.