Switch to using expr() function and make more interesting prpblem
Fix solution entry box and make better range of problems
Write function to simplyify terms in sum of expressions
Split into separate + and - exercises to make it easier to deal with hints
Rewrite rate problem hints
Include a solution box for the domain of the expression
Remove redundant variable
Split final hint
Remove u and v from array of possible variables
Make sure exercises use randVar when picking a random variable
Merge pull request #43434 from petercollingridge/randvar2

fix #43236; fix #43385;

Test plan: check each exercise still displays an equation

Auditors: beneater, cbhl
Change method of generating problem to avoid questions that simplify
Simplify fraction where required
Add solution
Remove unnecessary variables
Avoid the rare chance that the two constants could be 0 and the coefficients share a factor
Fix hint error
Add hint plural
Make numbers nicer
Minor typo

'isoceles' to 'isosceles'
fix #41775;

Test plan: Check my spelling

Auditors: beneater, alpert