Question

S=1+7+13+...+151
Ma ajutați va rog ​

Answer 1

Este o progresie aritmetica cu ratia 6.

$Sn=\frac{(a_1+a_n)\times n}{2} \\\\a_n=151\iff 151=1+(n-1)\times 6\\6n-6+1=151\\\\6n=156\\n=26$

Deci termenul 151 are rangul 26.

$S_{26}=\frac{(a_1+a_{26})\times 26}{2} \\S_{26}=\frac{(1+151)\times 26}{2}\\S_{26}=152 \times 13 \\S_{26} = 1976$