Question

determine o vigésimo termo da P.A (1,8,15...)

Answer 1

an = 1 + (20-1)7
an = 1 + 19.7
an = 1+ 133
an = 134

Answer 2

$a1=1\\a2=8\\a2-a1=r\\8-1=r\\r=7\\an=a1+(n-1).r\\a20=1+(20-1).7\\a20=1+19.7\\\\\boxed{a20=1+133=134}$