Respostas

2013-04-25T22:25:38-03:00

7)
a21 = a1 + 20r
-40 = 100 + 20r
20r = -40 -100
20r = -140
r = -140/20
r = -7

simples (:

9 3 9
2013-04-25T23:52:06-03:00

an=a1+(n-1)r

-40=100+(21-1)r

-40=100+20r

-40-100=20r

-140=20r

-140/20=r

-7=r

3 4 3