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r=5
an=a1+(n-1)*r
an=-6+5n-5
an=-11+5n
Sn=(a1+an)*n/2
(-6-11+5n)*n/2 =657 /*2
5n² -17n-1314=0
Δ=b²-4ac
Δ=289+26280 =26 569
√Δ=163
n1=(-b+√Δ)/(2a) =(17+163)/10 = 18
n=18
n2=(-b-√Δ)/(2a) < 0 nie dotyczy