Question

Rezultatul calcului :  1+4+7+10+.....+139 =  ?

Answer 1

1+4+7+10+…+139=?
r=3
$a_{1} =1$
$a_{n} =139$
$a_{n} = a_{1} +(n-1)*r$
1+(n-1)*3=139
3*(n-1)=138
n-1=46
n=47
$S_{n} = \frac{( a_{1}+ a_{n} )*n}{2}$
$S_{n}= \frac{(1+139)*47}{2}$
$S_{n}=3290$