Question

S=1+11+21+31+......+110
ma ajuta cineva?

Answer 1

E gresit ultimul termen, trebuie sa fie 101.

a₁=1
r=10
$a_{n}$ = 101
n=11 termeni 

Cum am aflat n?

$a_{n}$ = 101
a₁+ (n-1)·r = 101
1+ (n-1)·10 = 101
(n-1)·10 = 100
n-1=10
n=11

S=$\frac{(a_{1}+a_{n})*n}{2} = \frac{(1+101)*11}{2} = \frac{102*11}{2} =51*11=561$

Imi votezi ca cel mai bun raspuns?