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a fost răspuns

Dati factor comun, apoi calculati:
a) 10+20+30+...+800
b) 13+26+39+....+715
c) 21+42+36+.....+1890
d) 101+202+303+.....+909+1010+......+9898+9999

Exemplu :

10+20+30+....+800=10*1+10*2+10*3+...+10*80=10*(1+2+....+80)=10*80*81:2=32400

Răspuns :

10 + 20 + .. + 800 =

= 10(1 + 2 + .. + 80)

= 10×80×81÷2

= 32400

13 + 26 + .. + 715 =

= 13(1 + 2 + .. + 55)

= 13×55×56÷2

= 20020

21 + 42 + .. + 1890 =

= 21(1 + 2 + .. + 90)

= 21×90×91÷2

= 85995

101 + 202 + .. + 9999 =

= 101(1 + 2 + .. + 99)

= 101×99×100÷2

= 499950

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