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1+2+3...+20, 1+2+3...+200, 1+2+3...+21???

Răspuns :

1+2+3+........+20
=20×(20+1):2
=20×21:2
=420:2
=210
1+2+3+......+200
=200×(200+1):2
=200×201:2
=40200:2
=20100
1+2+3+......+21
=21×(21+1):2
=21×22:2
=462:2
=231