linear equations

Which of the following equations is/are linear in the variables x, y, zx,\ y,\ z?
i.) 10x+20y2+30z3=010x+20y^2+30z^3=0
ii.) 2xy3=112z\sqrt{2}x-\frac{y}{3}=-1-\frac{1}{2}z
iii.) 5xy+z=0\sqrt{5x}-y+z=0
iv.) x=6x=-6
v.) xyz=5xy-z=5
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