Exercise Which of the following are (systems of) linear equations?

`x`_{1}^{2} + `x`_{2}^{2} = 1

`x` - `y` = `y` - `z` =
`z` - `x`

`x`_{1} + `x`_{2}
= `x`_{3} + `x`_{4} + 1, `x`_{1} + `x`_{3}
= `x`_{2} + `x`_{4} + 2`, x`_{1} + `x`_{4}
= `x`_{2} + `x`_{3} + 3

`xy` = 1

`u` + `v` + `w` = 2^{1/2},
`u`^{2} + `v`^{2} + `w`^{2} = 3^{2/3}