Question

Determinati n apartine lui N,n$\geq$2,pentru care combinari de n luate cate 1 +combinari de n luate cate 2=15


repede va rogggg

Answer 1

combinari de n luate cate 1 +combinari de n luate cate 2=15

n!/(n-1)! +n!/[2!(n-2)!]=15

n(n-1)!/(n-1)!+n(n-1)(n-2)!/[2(n-2)!]=15

n+ n(n-1)/2=15

2n+n²-n=30

n²+n=30

n(n+1)=30

=> n=5