Respostas

2013-05-26T15:34:48-03:00

an=a1+(n-1).r
a21=6+(21-1).r
66=6+20r
66-6=20r
60=20r
r=3 

1 5 1
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2013-05-26T15:35:35-03:00

An = a1 + (n-1) . r

66 = 6 + (21-1) . r

66 - 6 = 20r

20r = 60

r = 60 / 20

r = 3

2 5 2