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S3=41+42.......... +80

S34142 80 class=

Răspuns :

S3=41+42+...+80
Observăm că lipsește 1+2+3+...+40

1+2+3+...+40=40(40+1):2=40×41:2=1640:2=820

S3=41+42+...+80-820
S3=80(80+41):2-820
S3=9680-820
S3=8860