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Iiiiiiii
a fost răspuns

Calculati urmatoarele sume :
a) 1×2+2×3+3×4+....+8×9+9×10=
b) 10×20+20×30+30×40+40×50+......+90×100=
Dau si coronita

Răspuns :

Antonelachetrean
a)2+6+12+20+30+42+56+72+90=330
b)200+600+1200+3000+4200+5600+7200+9000=33000
Getatotan
a) 1×2+2×3+3×4+....+8×9+9×10= 2 +6 + 12 +20 +30 + 42 +56 +72 +90 = 
   = 330 

b) 10×20+20×30+30×40+40×50+......+90×100=
 =  10 ·10 · 1·2  + 10·10 ·2·3 +   .... + 10·10 ·9 ·10 = 
 = 100 ( 1· 2 + 2·3 + ... + 9·10 ) = 
  = 100 · 330 = 33000

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