Let"z_j =x_j +i_j," where "xj,yj" are real number=1,2,3.
(a) We have
"z1 +z2 =(x1 +x2)+i(y1 +y2)=(x2 +x1)+i(y2 +y1)=z2 +z1." "z1z2 =(x1x2 \u2212y1y2)+i(x1y2 +x2y1)=(x2x1 \u2212y2y1)+i(y2x1 +y1x2)=z2z1."
is valid for real numbers and so it holds for the real and imaginary parts of"z_1z_2,z_3."
Consequently we have
"(z_1 +z_2)+z_3 =z_1 +(z_2 +z_3)."
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