Question

Determine o 12 termo da P.A ( 3,5,7)

Answer 1

an = a1 + (n-1).r
an= 3 + (12-1).2
an=3 + 11.2
an=3+22 = 25 

Espero ter ajudado :) 

Answer 2

$a_{1}=3 \\ r=2 \\ \\ a_{n}=a1+(n-1).r \\ a_{12}=3+(12-1).2 \\ a_{12}=3+11.2 \\ a_{12}=3+22 \\ \left[a_{12}=25]$