I have several long formulae, each formula occupies almost the whole width of the line:
$$ 1 * 2 + 3 * 4 + 5 * 6 + ( 7 * 8 ) \tag{a}$$ $$= 1 * 2 + 3 * 4 + 5 * 6 + 7 * 8 \tag{b}$$ $$<> 1 * 2 + 3 * 4 + 5 * ( 6 + 7 ) * 8 \tag{c}$$
Then I would like to add "$b <> $e" and "$e = f$":
$$ 1 * 2 + 3 * 4 + 5 * 6 + 7 * 8 \tag{b}$$ $$<> 1 * 2 + 3 * ( 4 + 5 ) * 6 + 7 * 8 \tag{e}$$ $$= 1 * 2 + 3 * 9 * 6 + 7 * 8 \tag{f}$$
But here I have repeated $(b)$... Does anyone have an idea to represent all these relations without repeating any formula?