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A melhor resposta!
2014-08-27T17:55:59-03:00
Q=5              an=a1+(n-1).r
a2=12          12=a1+(2-1)5
a1=?             12=a1+1.5
                       12=a1+5
                        a1=12-5
                          a1=7
  logo: P.A(7,12)     
                            
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