Question

Se considera suma S=1+2+3+...+n.n apartine N.Determinati n,daca n+1 reprezinta 20% din S

Answer 1

S=1+2+3+...+n
S=n(n+1)/2
n+1=20/100 x S => n+1=2/10 x n(n+1)/2 (2 se simplifica cu 2) => 
n+1=n(n+1)/10 (10 se trece in partea cealalta cu semn schimbat) => 
10(n+1)=n(n+1)║:(n+1) => n=10