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a1=1
r=-2
an=1+(n-1)*(-2)
an=1+-2n+2
an=-2n+3
Sn=-15
-15=1-2n+3/2 *n /*2
-30=n-2n²+3n
2n²-4n-30=0 /:2
n²-2n-15=0
Δ=4+60=64
√Δ=8
n₁=5
n₂=-3odpada
n=5 5 wyrazów dodano
a5=-10+3
a5=-7