Math, I love it. It has always been my favorite subject and I have always done well in it... except for one calculus class in college. During this time I actually learned a simple method of learning math well. My grades on my test were declining from a A to a D one letter per test. Yes I was going out a lot and spending too little time studying, but during my attempt to correct my grades I learned that for me learning math was about practice. I could easily understand the problems, but I had trouble remembering the formulas and solutions unless I did the homework routinely. My next calculus class I was assigned the odd problems for homework, but decided to do all of them instead, since they were not too time consuming. My test showed the results. 93% was my lowest score on any test. I learned for me the more I practiced the better I remembered the material. Math is a little different than other subjects. It is not subjective at all. It is based on fact. It is mostly about memorization. Word problems can often help you understand math better giving a practical side to the logic, but remorizing formulas is more about practice and studying.
You should not. That is what a "cheatsheet" is for.
Mathematics is not about memorization of formulas. Unfortunately that is the (wrong) idea that schools tend to push onto children.
Maths is very much a creative subject and it is about coming up with new ideas. Memorizing formulas will not help your child. Instead encourage him/her to solve problems on their own and never be afraid to make mistakes. Many children have unique ways of looking at the world and solving problems...encourage your child to work things out their own way.