Respostas

2014-03-06T18:55:34-03:00
a20 = -100
a1 = r


an = a1+(n-1)r
-100 = a1 + (20-1)r 
-100 = a1 + 19a1 
-100 = 20a1 
-100 / 20 =a1 
a1= -2 

Se a1 = -2 , logo r=-2

Sn = \frac{(a1+an)n}{2}
S20=(-2-100)10 / 2
S20= -102 x 10 / 2
S20= - 510
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