A question about proportionality

[a.s.h.]

If I say that "y is proportional to x", does that mean y = kx (where k is a non-zero constant)?

Or would I strictly need to say "y is directly proportional to x" to mean y = kx?

[joe]

Good question! Is there a difference between 'proportional' and 'directly proportional'?

Thinking about the way I use it, my answer is this:
y proportional to x means y = kx, k constant, as you say.
y inversely proportional to x means y = k/x.

I think that I only ever say 'directly proportional' when I want to distinguish it from inverse proportionality: 'y is inversely proportional to x but directly proportional to z', i.e. y = kz/x.

Joe

[a.s.h.]

I see, thanks.

Yes, that is also how I see it. I think personally it's redundant to have "directly" proportional, unless one wants to emphasise between inversely proportional, as you said.