Question

hei! cum se rezolva :
1+5+9+13+...+x=190

Answer 1

1 5 9 13 ...... x sunt in progresie aritmetica
r=5-1=4
suma Sn =(a1+an)*n/2=(a1+a1+(n-1)r)*n/2=190
(1+1+(n-1)*4)*n/2=190
(2+4n-4)n/2=190
(4n-2)n=380
4n2-2n-380=0 / :2
2n2-n -190=0
delta=1521
n1 2=1+-39 / 4
adica n1=-19/2 nui poyitiv
n2=10

deci n=10
a10=a1 +9r=1+9*4=37
x=37