Explanation:
what you did was you multiplied all numbers (incl. pi as number) :
1/6, pi, 1/12, pi
and that gives 0.137077839. no i or pi attached. just the plain result number.
so, if you need to just calculate the factors of pi, but don't need the fully numeric result, then your problem looks like
1/6 × pi × 1/12 × pi = 1/72 × pi²
so, you have an expression of pi² with a factor of 1/72.
you only need to type in all the factors, not pi itself (that's what you need to add at the end yourself).