Question

calculati suma primilor 20 termeni in progresie aritmetica cu proprietatea a6+a9+a12+a15=30

Answer 1

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

$a_{6}+ a_{15}=30:2=15= a_{1}+ a_{n}$

$S_{n}= \frac{( a_{1}+ a_{n})n }{2}$

$S_{20}= \frac{15*20}{2}$

$S_{20}=150$