Question:
If 15C3r = 15Cr + 3, find r.
Solution:
Given;
${ }^{15} C_{3 r}={ }^{15} C_{r+3}$
$15=3 r+r+3 . \quad\left[\because{ }^{n} C_{x}={ }^{n} C_{y} \Rightarrow x=y\right.$ or, $\left.n=x+y\right]$
$\Rightarrow 15=4 r+3$
$\Rightarrow 4 r=12$
$\Rightarrow r=3$
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