(an)-арф.прогрессия a3=0 a8=25 a6,S6-?

Question

(an)-арф.прогрессия
a3=0
a8=25
a6,S6-?

Answer 1

A1+2d=0
a1=-2d
a1+7d=25
-2d+7d=25
5d=25
d=5
a1=-2*5=-10

a6=a1+5d=-10+5*5=-10+25=15
S6=2a1+5d/2 * 6 = (-20+25)3=5*3=15

Answer 2

{a1+2d=0⇒a1=-2d
{a1+7d=25
-2d+7d=25
5d=25
d=25:5
d=5
a1=-2*5
a1=-10
a6=a1+5d=-10+5*5=-10+25=15
S6=(2a1+5d)*6/2=(-20+25)*3=5*3=15