Question

sabendo que a razao e 5 e a16=80 calcule o primeiro termo

Answer 1

r = 5 

$a_{n} = a_{1}+(n-1).r$, logo:
$a_{16} = a_{1}+(16-1).5$
$80 = a_{1}+75$
$80-75 = a_{1}$
$a_{1}$= 5

Answer 2

$\boxed{a_n=a_1+(n-1)*r}\\\\ a_1_6=a_1+15r\\\\\ 80=a_1+15*5\\\\ 80=a_1+75\\\\\ a_1=80-75\\\\ \boxed{\boxed{a_1=5}}$