If ^ 20 C_ r+1 = ^ 20 C_ r-1 , then r is equal to - Vidyadip

MCQ

If ${ }^{20} C_{r+1}={ }^{20} C_{r-1}$, then $r$ is equal to

A
$19$
✓
$10$
C
$12$
D
$11$

✓

Answer

Correct option: B.

$10$

$r +1+ r -1=20\left[\because{ }^n C_x={ }^n C_y \Rightarrow n = x + y\right.$ or $\left.x = y \right]$
$\Rightarrow 2 r =20$
$\Rightarrow r =10$

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