Merge pull request #55 from Dick-Lane/master

cosmetic: displaystyle for rational function, different size parentheses
New lin alg problem.
New.
Small cosmetic change.
Update to correct occasional error, improve formatting and randomization, and use parserPopUp instead of string compare.
New lin alg problem.
Lin alg.
Minimal change so the "answer" does not contradict the constraint.
On p540 of SRW-5, the corresponding problem is 23 [without constraint].

An alternative revision would just remove the first clause, leaving
   If \sin(x - pi/2) = A \, cos(x), then A = [box].