Question
Prove thart.
$\frac{1}{9!}+\frac{1}{10!}+\frac{1}{11!}=\frac{122}{11!}$ 

Answer

 $\text{L.H.S.}= \frac{1}{9!}+\frac{1}{10!}+\frac{1}{11!}$

$=\frac{1}{9!}+\frac{1}{10\times9!}+\frac{1}{11\times10\times9!}$ 

$=\frac{11\times10+11+1}{11\times10\times9!}$

$=\frac{110+11+1}{11!}$

$=\frac{122}{11!}$

$\text{R.H.S.}$

Hence, $\frac{1}{9!}+\frac{1}{10!}+\frac{1}{11!}= \frac{122}{11!}$

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