Question

Dada a P.A.(9,7,5,...)encontre o nono termo

Answer 1

a₉ = a₁ + 8r
a₉ = 9 + 8*(-2)
a₉ = 9 - 16
a₉ = -7 

Answer 2

Razão : 

$\mathtt{R = A_2 - A_1} \\ \mathtt{R = 7 - 9} \\ \mathtt{R = -2}$

$\mathtt{A_n = A_1 + (n-1)~.~R} \\ \mathtt{A_{9}= 9 + (9 -1)~.~(-2)} \\ \mathtt{A_9 = 9 + 8~.~(-2)} \\ \mathtt{A_9 = 9 -16} \\ \mathtt{A_9 = -7}$