Personally, I do not like the name of 'math cheat sheets', as it implies some level of wrong doing. I feel there is nothing wrong with using them ... just don't take them into a test with you!
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Cheats sheets are a great idea because they contain ALL the information on a topic in one small concise space that a student has already studied and learned. They typically contain formula information, or brief explanatory notes on the topic at hand. These should not be used when teaching a topic, but are excellent when doing homework, or as study and revision aids.
A geometry cheat sheet could contain images of the different 2D and 3D shapes, with their names and structure labeled. It would also contain the formulae required to establish area, perimeter and volume of these shapes. It would have a small explanation of the Pythagorean Theorem, along with it's formula, a diagram of geometric quadrants and their labels; and much more!
As you can see, there should be a huge amount of information on a cheat sheet. For this reason, you wouldn't give your child a cheat sheet until most or all the information it contains has been taught and understood. The ultimate goal is that they remember all these rules and formulae - but as they study and practice, you don't want them spending all their time 'looking up' the information they can't remember. That is not a productive use of their time.
You will most probably find, just as 'way back when' they were learning their math facts - they will use the print out to help them. Slowly but surely, with use and practice of the information, they will use the cheat sheets less and less - because with constant use of the rules, they will start to remember them!
Algebra Cheat Sheet
Geometry Cheat Sheet
Numbers Cheat Sheet
Measurement Cheat Sheet
Graphs Cheat Sheets
If you have not covered all the information on a full cheat sheet, but want to give your student some assistance. Rather than give them a cheat sheet, consider giving them a math poster that covers the information. Sticking with my earlier example, if you child is studying geometry and has just started to study 3D Shapes - consider printing off some of my math posters that only show 2D Shapes and 3D Shapes. This is all the information they need right now!