"$a1", because you are showing mixed numbers, you get "1 1/2" as the string passed to Compute(). But the space is implicit multiplication, so you compute effectively "1*1/2", which is 1. If you want to process mixed numbers (not just show them), you need to include the allowMixedNumbers=>1 flag as well.It is not clear to me why you are doing the
Compute() at all, since $a1 is already a MathObject. There is no need to recompute it that I can think of.OOPS! I see Alex already answered. Sorry for the noise.