16.Aplici sumele lui Gauss
a=1+2+3+...+40=40*41/2=820
b=2+4+...+80=2*(1+2+...+40)=2*820=1640
2a-b=2*820-1640=0
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17.adui cele 2 egalitati
a+2b+3c=41
5a+4b+3c=55
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6a+6b+6c=96 6(a+b+c)=96
a+b+c=96:6=16