Question

determine o vigesimo termo da pa (1,8,15,...)

Answer 1

$pa (1,8,15,...)\\\\\\ a_n=a_{20}=?\\a_1=1\\ n=20\\ r=15-8=7\\\\\\ a_n=a_1+(n-1)*r\\\\\\ a_{20}=1+(20-1)*7\to \\\\ a_{20}=1+19*7\to \\\\ a_{20}=1+133\to \\\\ a_{20}=134$

Answer 2

Termo geral:
An=A1+(n-1)r
      
 A  P.A. é (1,8,15)
 8-1=7, 15-8=7, então a razão r é  7 
logo,
A20=1+(19)7
A20=1+133=134

o vigésimo termo é 134