Question

sa se determine ratia si suma primilor 15 termini intr-o progresie aritmetica daca a3=12 si a4=16

Answer 1

r=a4-a3=16-12=4
a3=a1+2*r
a1=a3-2*r
a1=12-8=4
$S_{15}= \frac{ (a_{1}+ a_{15})*15 }{2}$
a15=a1+14*r=4+14*4=4+56=60
$S_{15}= \frac{(4+60)*15}{2}= \frac{64*15}{2}=480$0