Question

1+3+5+.............+x=100 Ajutati-ma va rog!!! P.S. Suntem la lectia cu progresii

Answer 1

Observăm că rația este 2 => Sn = (a1 + an)•n/2 => 100 = (1 + x)•n/2; an = x = a1 +(n-1)•r = 1 + 2(n-1) = 1 + 2n - 2 = 2n - 1 => 100 = (1 + 2n - 1)•n/2 => 200 = 2n^2 => n = 10 => x = 200/10 -1 = 19

Answer 2

1+3+5+.............+x=100
an=x
an=2n-1
2n-1=x
2n=x+1
n=(x+1) /2

1+3+5+.............+x=100
1+3+5+.............+x= n²;  n=(x+1) /2
[(x+1) /2]²=100
(x+1) /2=√100
(x+1)/2=10
x+1=20
x=20-1
x=19

R: 1+3+5+.....+19=100