Question

Qual é o vigésimo termo da PA (9,13,17,21...)

Answer 1

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

$a_{20}=9+(20-1)\cdot 4$

$a_{20}=9+19\cdot4$

$a_{20}=9+76$

$\boxed{\boxed{ a_{20}= 85}}$