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S= ((-20+19*3)/2)*20
S=(37/2)*20=370
Z twojego wzoru
Sn=((a1+an)/2)*n
S20=((a1+a20)/2)*20
/ a20=a1+19r
/ a20=-10+19*3
/ a20=57-10=47
--> S20=((-10+47)/2)*20=370
Robisz gdzieś błąd w rachunkach.