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,,2+4+6+8+10+12+14+16+18=100"
,,3+5+7+9+11+13+15+17+19"

VA ROG DAU COROANA!


Răspuns :

2+4+6+8+10+12+14+16+18=
= 2×(1+2+3+4+5+6+7+8+9) =
= 2 × 9×10 : 2 =
= 90

3+5+7+9+11+13+15+17+19 =
= (19 + 3) × 9 : 2 = 22×9:2 = 11×9 = 99

9 reprezinta cati termeni are suma.
Am impartit la 2 deoarece termenii sumei sunt din 2 in 2.