[tex]\\r=6-12=-6 \\S_n=\frac{2a_1+r(n-1)}{2}*n \\S_{20}=\frac{2*12-6*19}{2}*20=(24-114)*10=-900[/tex]
a1 = 12
a2 = 6
S20 = ?
n = 20
r = a2 - a1 = 6 - 12 = -6
an = a1 + (n - 1) * r
a20 = 12 + (20 - 1) * (-6)
a20 = 12 + 19 * (-6)
a20 = 12 - 114
a20 = -102
S20 = [ (a1 + a20) / 2 ] * n
S20 = [ (12 + (-102)) / 2 ] * 20
S20 = [(-90) / 2 ] * 20
S20 = -45 * 20
S20 = - 900 ----------- odpowiedz