Question

Qual é o vigésimo termo da P.A (-10, -8, -6, ...)

Answer 1

 A20 = -10 + ( 20 -1 ) . 2
A20 = -10 + 19 . 2
A20 = -10 + 38
A20 = 28           ok

Answer 2

$a_n=a_1+(n-1)\cdot r$

$a_{20}=-10+(20-1)\cdot2$

$a_{20}=-10+19\cdot2$

$a_{20}=-10+38$

$\boxed{\boxed{a_{20}=28}}$