[tex] x=1+2+3+...+20=10*21=210\\ \\ y=2(1+2+3...+20)= 2x=420\\ \\ z=3(1+2+3+...+20)=3x=630\\ \\ x+y+z=6x=1260\\ \\ \\ Am~folosit~formula~lui~Gauss:~1+2+3+...+n=\frac{n(n+1)}{2}. [/tex]
A) X= 20×21 ÷ 2X= 420 ÷ 2X=210 Y=2+4+6+...+40 |÷2Y=1+2+3+...+20Y=420 Z=3+6+9+...+60 |÷3Z=1+2+3+...+20Z=420 ×3 ÷ 2Z=1260 ÷ 2Z= 630B)210+420+630=1260
