Question

determine numa pa o 8º,dados a12=25 e r =3

Answer 1

$a_{n} = a_{1} +(n-1)r$

$r=3 \\ \\ a_{12} =25 \\ \\ a_{12} = a_{1} +(12-1).3 \\ \\ 25= a_{1} +33 \\ \\ a_{1} =25-33 \\ \\ a_{1} =-8 \\ \\ \\ a_{8} =-8+(8-1).3 \\ \\ a_{8}=-8+7.3 \\ \\ a_{8}= -8+21 \\ \\ a_{8} =13$