Which of the following equations is/are linear in the variables x, yand z? x-y…

Which of the following equations is/are linear in the variables x, yx,\ yand zz?
  1. xy+2z=0x-y+2z=0
  2. x2+2y2+3z2=1x^2+2y^2+3z^2=1
  3. 1x +1y1z=2\frac{1}{x}\ +\frac{1}{y}-\frac{1}{z}=2
  4. xyz=1xyz=1
  5. 2xy=5z2x-y=\sqrt{5}-z
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