Question

Calculati suma primilor trei termeni a unei progresii aritmetice (an)n>1, stiind ca a1=1 si a3=5

Answer 1

Sn=((a1+an)n)/2 si aplici pt S3 =((a1+a3)3)/2=9

Answer 2

a3=a1+2r => 5=1+2r  => 2r=5-1  =>  2r=4  => r=2

a2=a1+r=1+2=3
 
a1=1, a2=3, a3=5

S=a1+a2+a3=1+3+5=9 => S=9

sau

$S= \frac{n(a1+ a_{n}) }{2}= \frac{3(1+5)}{2}= \frac{18}{2}=9$