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r=-1/2
an=a1+(n-1)*r
a10=16 -9*1/2 =16 -4,5 =11,5
a20=16 -19*1/2 =16-9,5 =6,5
n=20-9 ( dziesiaty wyraz juz należy do sumy)
n=11
Sn=(a1+an)*n/2
S(10-20) =(11,5+6,5)*11/2 =18*11/2 =9*11
S(10-20)=99