Question

Dada a p.a (-20, -12, -4 ...) Calcule a10+a20?

Answer 1

 r = -12 - ( -20 )  
Razão = r  =  8

==

an =   a1 + ( n -1 ) . r
a10 =  -20 + ( 10 -1 ) . 8
a10 =  -20 + 9 . 8
a10 =  -20 + 72
a10 =  52

===

an =   a1 + ( n -1 ) . r
a20 =  -20 + ( 20 -1 ) . 8
a20 =  -20 + 19 . 8
a20 =  -20 + 152
a20 =  132

===

x = a10 + a20

x = 52 + 132

x = 184