Question

obtendo o 18 da PA ,tal que a1=4 a2=1

Answer 1

$\textsf{Resolu\c{c}\~ao~da~quest\~ao, veja~como~se~procede}:\\\\\\\\ \textsf{Determinar~a~raz\~ao~da~P.A.}\\\\\\\ r = {a_{2}}-{a_{1}}\\\\ r =1-4\\\\\ r = -3\\\\\\\\\ \textsf{Organizando~os~dados~teremos~que}:\\\\\\ a_{1} = 4\\\\ r = -3\\\\ n = 18\\\\\\\ \textsf{Aplicando~na~f\'ormula~geral~da~P.A.~temos~que}:\\\\\\\ a_{n} = a_{1}+(n-1)~\cdot~r\\\\\ a_{18} = 4+(18-1)~\cdot~(-3)\\\\\ a_{18} = 4+ 17~\cdot~(-3)\\\\\ a_{18} = 4+(-51)\\\\\ a_{18} =4-51\\\\\\ \large\boxed{\boxed{\boxed{a_{18} = -47.}}}}}}}}}}}}$

Espero que te ajude. :-). 

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