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Ajutor!!Combinari de 2 luate cate n+2.
n=n+2
k=2​

AjutorCombinari De 2 Luate Cate N2nn2k2 class=

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Targoviste44

[tex]\it C_{n+2}^2=21 \Rightarrow \dfrac{(n+2)!}{2!(n+2-2)!}=21 \Rightarrow\dfrac{n!(n+1)(n+2)}{2\cdot n!}=21 \Rightarrow\\ \\ \Rightarrow (n+1)(n+2)=42=6\cdot7 \Rightarrow n+1=6 \Rightarrow n=5[/tex]

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