Is copying equations plagiarism?

Mathematical equations are not normally considered subject to plagiarism, since there are only so many ways they can be written, and because they represent ideas rather than material that can be "stolen" without proper attribution.

So long as you cite the material you are drawing from, and explain the equations in your own words, you should be fine.

I do believe, like @aeismail and other commentators, that equations are knowledge, and not subject to accusations of plagiarism. Yet, you should be cautious about your mathematical notations and their correspondence with the copied equations.

If you have adapted the proofs, or the calculations, with your own symbols, and typed the equations yourself, you are fine.

Be cautious about some mathematical notations (imaginary unit, convolutions, ordinal variables, greek letters), that may be more or less standard depending on the scientific field. For instance, the imaginary unit is often "i" in maths, and "j" in electrical engineering. The reuse of other field's notations should not look too artificial.

However, I have seen persons who have cropped blocks of equations from some source in slides or reports, and who did not bother change their notations or cite the source. This behavior is closer to laziness or dishonesty than to a genuine personal work. But working on equations, their variables, to make them more understandable, clearer, is a plus.

Finally, one side of plagiarism is in automated plagiarism detection tools. You may be worried about texts, but so far, they do not seem very efficient with equations. Unfortunately, since it is difficult to search formulae online.

You should remember that academic plagiarism is not plagiarism in the sense of copyright. Academic plagiarism is mainly the "stealing" of other people's ideas. As such rephrasing is normally not enough. Mathematical formulas are a bit of a gray area in that. Much of it is (very) well known and also subject to possible independent invention. Once you have a lot of them though, the probability of the same approach without awareness becomes unlikely.

In all cases of gray areas, especially if they concern aspects of professional ethics (such as plagiarism), be safe and reference. Also be aware that not all readers are familiar with all parts of maths and may not recognise that you are just describing a reasonably well-known concept, not introducing it yourself (I have reviewed a journal paper with this issue, directing the authors to add the reference was sufficient, it was clear that they actually didn't come up with the idea).