Question

Numarul submultimilor cu cel mult doua elemente ale multimi A={1,2,3......n} este egal cu 11 Determinati nr natural n

Answer 1

C.E n≥2

Cn luate cate 0+Cn luate cate1+Cn luatecate 2=11
1+n+n(n+1)/2=11
(n+n²)/2=10
n²+n-20=0
n1=-5, nu convine
n2=4, convine

verificare
1+4+6=11 ,
11=11adevarat, bine rezolvat