Given two terms of the arithmetic​ sequence, a3=25 and a8=−10​, find a1 and d.

Question

Given two terms of the arithmetic​ sequence, a3=25 and a8=−10​, find a1 and d.

Answer 1

If you make a little sketch, it's easy to see that if you add 5d to 25 you get -10:

So 25 + 5d = -10

5d = -35

d = -7

So to get a₁, just add 7 twice

25 + 7 + 7 = 39