Question

nCr-1=nC3r find r From combinations

Answer 1

The solution is attached as a word file

fcbef0ef46a7d109eba78f292ed9c1a5.doc

Answer 2

Given:

$\bold{nCr-1= nC3r}$

To find:

r=?

Solution:

$\to nCr-1=nC3r \\\\ now, \\\\nCr-1=nCn-r+1\\\\ so,\\\\ nCn-r+1=nC3r \\\\hence,\\\\ \to n-r+1=3r\\\\ \to n=4r-1$

$\bold{\\ \to r= \frac{n+1}{4}}$