Question

Sa se determine n€N astfel incat f(1)+f(2)+....+f(n)=105

Answer 1

f(1)+f(2)+f(3)+...+f(n)=105
Consideram
f(1)=1
f(2)=2
f(3)=3
....
f(n)=n astfel incat
1+2+3+...+n=105
n(n+1)/2=105
n(n+1)=210
n(n+1)=210⇔14·15=210⇒n=14