When I want to refer to an equation similar to this image, what term should I use? Is it correct to say "Equation 3", although the equation contains a proportionality symbol?

enter image description here

closed as unclear what you're asking by FumbleFingers, Nathan Tuggy, joiedevivre, Michael Rybkin, Varun Nair May 7 '18 at 7:06

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    It's not clear what you're asking. – Michael Rybkin Apr 29 '18 at 13:30
  • Honestly, this seems like a question for Mathematics, since it's not about everyday English but technical terminology in a field that's specialized enough that most native speakers not only do not know which to use but can't easily figure out or check. – Nathan Tuggy Apr 29 '18 at 14:01
  • This question is about terminology specific to math. Although we may have people who could answer it, none have volunteered an answer. I think the question would get more traction on a different SE site. – joiedevivre May 5 '18 at 4:51
  • @NathanTuggy while the language is mathematical it's not esoteric. Most students learn about proportions and equations very early on. – Andrew May 5 '18 at 16:28

Yes, you can use equation when you reference this expression.

According to Wikipedia, an equation is always a statement of equality. Equality is an example a relation. Inequality, another relation, has the separate term inequation for a statement that describes it - but that term is somewhat uncommon (it is a dictionary word but my spell checker is unaware if it).

For the proportionality relation you can use the word proportion.

Other relations, such as congruence, membership, inclusion, or orthogonality, don't have specific names for statements describing them.

However, in technical writing, an equation is a part of the text that describes a statement in symbolic language, often with a label that can be used for reference (such as "equation 1-32"). The statement can be about any relation between things, not necessarily equality. It can also describe inequality, congruence, proportionality, membership, inclusion, orthogonality, and other relations.

If you want to be specific about the kind of relation you can also refer to it with more specific terms. For example, the Triangle inequality can be written as

||x|| + ||y|| > ||x + y||    (1-32)

And then you can refer to it as Equation (1-32) but also as Inequation (1-32). Note however that most technical writing tools will enforce using the same term in cross references - so I would say using "inqeuation" in technical text is rare.

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