I have a long expression of the form
-(1/(12 M (2 + M - M ζ^2 - η^2))) (-2 + M (-1 + ζ^2) + η^2)(some long expr)
The expression is a result of a Mathematica command. (Here's the expression itself, for the reference.) The long expression
(some long expr) is a polynomial in ζ and η, and, if simplified, the whole expression would also be a (product of) polynomial(s) — unlike its form above, which is a ratio of polynomials.
The first two factors can be simplified by
Simplify successfully, but, when applied to the full expression, it leaves them unchanged. I tried
FullSimplify on the full expression, but I couldn't wait until it finishes.
I'd like to direct Mathematica to simplify the first two factors to
1/(12 M), without touching the rest, so that the result would be
1/(12 M)(some long expr)
How can I achieve this?