Here, you are indicating that x x is the variable because of the x x appearing. Alternatively (and more commonly), you would write f: R → R f: R → R and f(x) = kx + m f ( x) = k x + m.
So, it is correct to write k, m, x ∈R k, m, x ∈ R, but k k and m m are fixed while x x is not. You need to indicate which are fixed and which are not. K m x 5 K&M products that will make your everyday touring life easier.
