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-3 - 9 = 6r
6r = - 12 /:6
r = -2
a3-a1 = 2r
a3 - 2r = a1
a1 = 9 -2*(-2)
a1 = 9 + 4
a1 = 13
a) wzór an
an=a1+(n-1)r
an = 13 + (n-1)*(-2)
an = 13-2n+2
an = -2n+15
b) S20
n=20
a20 = (-2)*20 + 15
a20 = -40 + 15
a20 = -25
S20 = (13-25)/2 * 20
S20 = -12/2 * 20
S20 = -6*20
S20 = -120