Get mathematica to simplify expression with another equation

Question

I have a very complicated mathematica expression that I\'d like to simplify by using a new, possibly dimensionless parameter.

An example of my expression is:

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Answer 1

K = a*b*t/((t+f)c*d);

FullSimplify[ K, 
 TransformationFunctions -> {(# /. t/(t + f) -> p &), Automatic}]

(a b p) / (c d)

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Corrected update to show another method:

EQ1 = a1 == (ton (B ton^2 + A^2 B ton (toff + ton) + 
        A (ton^2 + B (toff + ton)^2)))/(toff + ton)^3;

f = # /. ton + toff -> ton/p &;

FullSimplify[f @ EQ1]

a1 == p (A B + A^2 B p + (A + B) p^2)

I don't know if this is of any value at this point, but hopefully at least it works.