Question

Determine o 20° termo da pa (25, 17, 9, 1, -7, ... )

Answer 1

$PA(25, 17, 9, 1, -7, ... )\\\\\\ a_{1} =25 \\a_{n} = a_{20} =?\\n=20\\ r=-8\\\\\\ a_{n} = a_{1} +(n-1)*r\\\\\\ a_{20} = 25 +(20-1)*(-8)\to\\\\ a_{20} = 25 +(19)*(-8)\to\\\\ a_{20} = 25 -152\to\\\\ a_{20} = -127$

Answer 2

a1 = 25
a2 = 17
n = 20

r = a2 - a1
r = 17 - 25
r = - 8

PA 
an = a1 + (n - 1).r
a20 = a1 + (20 - 1).r
a20 = a1 + 19r

a20 = a1 + 19r
a20 = 25 + 19.(- 8)
a20 = 25 - 152
a20 = - 127

Resp.: a20 = - 127