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calculai 10+20+30+...+120

Răspuns :

=10(1+2+3+12)
=10((13x12):2))
=10x78
=780
10+20+30+...+120
simplificam ecuatia prin 10 si avem
10 x (1+2+3...+12)=10 x (12 x 13 /2)
=10 x (156/2)
=10 x78
=780