Question

gasti suma primilor 20 termeni a unei progresii aritmetice stiind ca : a indice 6+a indice 9+a indice 12+a indice 15=20

Answer 1

$a_{1}+ a_{n}= a_{k}+ a_{n-k+1}$

[tex] a_{1}+ a_{20}= a_{6}+ a_{15} [/tex]
$a_{1}+ a_{20}= a_{9}+ a_{12}$
$2( a_{1}+ a_{20})=20$
$a_{1}+ a_{20}=10$

$S_{20}= \frac{10*20}{2}$

$S_{20}=100$