Question

numa p.a de dez termos ,o ultimo termo e igual a 22 a razao e igual a 2 determine o primeiro termo e a soma dos termos

Answer 1

$n=10$

$a_{10} =22$

$r=2$

$a_{1}=$ ?

$S_{10}=$ ?

$a_{n}=a_{1}+(n-1).r$

$22=a_{1}+(10-1).2$

$22=a_{1}+9.2$

$22=a_{1}+18$

$a_{1}=22-18$

$a_{1}=4$

$S_{n} = \dfrac{( a_{1} + a_{n} ).n}{2}$

$S_{10} = \dfrac{( 4 + 22 ).10}{2} =26.5=130$