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].
Merge pull request #56 from Dick-Lane/master

Minimal change so the "answer" does not contradict the constraint.