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Annapandacorn00
a fost răspuns


1+2+3+...+100=?
2+4+6+..+100=?
1+4+7+10+...+139=?

Răspuns :

1+2+3+...+100=100(100+1) / 2=100x101 /2 = 5050
2+4+6+...100=2(1+2+3+...+50)=2x[50(50+1)/2]=2x1275=2550
1+4+7+10+...+139=(3x0+1)+(3x1+1)+(3x2+1)+(3x3+1)+...+(3*46+1)= 47+3x(0+1+2+3+...+46)=47+3x{0+[46x(46+1)]/2}=47x3x1081=152421

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