Question

determine o vigésimo primeiro termo da P. A (-6-9-12)

Answer 1

$PA~\{-6;-9;-12;...\}\\\\ a_1=-6\\ r=-9-(-6)\to~~ -9+6 = -3\\n=20\\ a_{20}=?\\\\\\a_n=a_1+(n-1)\times r\\ \\\\a_{20}=-6+(20-1)\times (-3)\to\\\\ a_{20}=-6+19\times (-3)\to\\\\ a_{20}=-6-57\to\\\\ \boxed{a_{20}=-63}$