Răspuns :
a)2+6+12+20+30+42+56+72+90=330
b)200+600+1200+3000+4200+5600+7200+9000=33000
b)200+600+1200+3000+4200+5600+7200+9000=33000
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
= 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