Say you are writing a mathematical modeling paper in a science journal. How do you write your variable and parameter definitions in paragraph form so that you keep active voice but do not sound repetitive. For example
"$x$ is the average rate of change of ..."
Variable name followed by "is" sounds correct, but this must be frowned upon due to capitalization issues. Perhaps you could write,
"The average rate of change of ... is [called] $x$"
This is passive voice. We tend to use in proofs,
"Let $x$ be the rate of change of ..."
But this gets repetitive fast if you are writing in paragraph form. My guess is that this is more of an issue for applied mathematicians writing for non-math audiences.