Question

 Sa se calculeze suma 1+4+7...+31 . 

Answer 1

Ti-am atasat un fisier cu problema rezolvata!

Answer 2

este o progresie aritmetica, si avem:
$a_{1}=1 \\ \\ a_{n}=31 \\ \\ r=3$

$S_{11} = \frac{ (a_{1}+ a_{n})*n }{2} \\ \\S_{11}= \frac{(1+31)*11}{2} \\ \\ S_{11}=\frac{32*11}{2} \\ \\ S_{11}=\frac{352}{2} \\ \\ S_{11}=176 \\ \\ a_{n}= a_{1}+r(n-1) \\ \\31=1+3(n-1) \\ \\ 31=1+3n-3 \\ \\ 31=3n-2 \\ \\ 3n=31+2 \\ \\ 3n=33 \\ \\ n=11$[/tex]