Respostas

2014-03-31T08:56:49-03:00
An = a1 +(n -1)r
a15 = 28 +(15 - 1).(-5)
a15 = 28 + (14).(-5)
a15 = - 42



sn = (a1+an)n /2
s15 = (28-42).15 /2
s15 = (-14).15/2
s15 = -210 / 2 
s15 = -105