The phrase "a complex equation that" is correct and usable in written English.
You can use it when referring to a specific mathematical equation that has multiple variables or components, often in a scientific or academic context.
Example: "The researchers presented a complex equation that models the behavior of the system under various conditions."
Alternatives: "a complicated formula that" or "a intricate calculation that".
There's a complex equation that can be used to describe essayist Sloane Crosley's work.
Ultimately, though, his success is a complex equation that includes aspects we can't understand.
How else to explain a 1998 episode of The Simpsons in which a guy who usually comes across as a doughnut-eating doofus stands at a chalkboard bearing a complex equation that prefigures the discovery of the Higgs boson.
Microbiological data, from whatever source, need to be incorporated as a variable in a complex equation that considers other patient characteristics to determine the likelihood of pneumonia.
To calculate your BMR, you have to use a fairly complex equation that takes into account weight, height, sex, and age.
However, once we admit a concept of the common good, and that such good might outweigh the maximization of private gain, haven't we admitted that profits are just one variable in the complex equation that determines human well-being?
Deciding when that no-longer-herself moment was, however, turned out to be a more complex equation that anyone in Bem's family anticipated.
A programmable calculator is designed to help with complex equations that involve a lot of repetitive steps.
Expressing such ideas through mathematics alone yields complex equations that befuddle nearly everyone.
Class, like mathematics, consists of complex equations that may shift with the substitution of different values for X and Y, but the equations themselves remain rigid and fixed.
As scientists improved their characterization of the intricate physics of each device, they developed complex equations that took into account subtle effects that were not apparent from coarse laboratory measurements.
