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Determina numarul natural n care verifica egalitatea n(n+1)(n+2)=990
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Răspuns :

n(n+1)(n+2)=990
(n^2+n)(n+2)=990
n^3 +2n^2 +n^2 +2n=990
n^3 +3n^2 +2n=990
n^3 +3n^2 +2n-990=0
n^3 -9n^2 +12n^2 -108n+110n-990=0
n^2 ×(n-9)+12n×(n-9)+110×(n-9)=0
(n-9)×(n^2 +12n+110)=0
n-9=0
n^2 +12n+110=0
n=9
(n nu apartine lui R)