1.

यदि `.^(18)C_(r)=.^(18)C_(r+2)` तो `.^(r)C_(5)` का मान ज्ञात कीजिए।

Answer» `.^(18)C_(r)=-.^(18)C_(r+2)`
या `.^(18)C_(18-r)=.^(18)C_(r+2)`
तब `18-r=r+2`
`2r=16` या `r=8`
अब `.^(r)C_(5)=.^(8)C_(5)=(8!)/(5!3!)`
`=(8.7.6(5)!)/(5!.3.2.1)=56`


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