Question

O 7 termo da PA (1,-1..)

Answer 1

r = - 2
a1 = 1
n = 7
an = a7 = ?

an = a1 + (n- 1 ) . r
a7 = 1 + ( 7 - 1) . ( - 2 )
a7 = 1 + ( 6 ) . ( - 2 )
a7 = 1 - 12
a7 = - 11

PA = ( 1, - 1, - 3 , - 5 ,- 7 , - 9, - 11 )

Answer 2

$(1,-1,\dots)$

$r=-1-1=-2$

$a_n=a_1+(n-1)r$

$a_7=a_1+(7-1)r$

$a_7=a_1+6r~~\Rightarrow~~a_7=1+6(-2)~~\Rightarrow~~a_7=1-12~~\Rightarrow~~a_7=-11$