Question: Variant to subs/algsubs ?

Hi,

I would like to format the solution given by maple.
I have the answer given by a paper and I would like to reproduce the same formula.

I have the expression :

(a1/2+a2/2)*ln(a1/2+a2/2+sqrt((a1/2+a2/2)^2+(b1/2-b2/2)^2+d^2))+(a1/2-a2/)*ln(a1/2-a2/2+sqrt((a1/2-a2/2)^2+(b1/2-b2/2)^2+d^2))

I would like to assume deltaa = a1-a2 and sigmaa = a1+a2.

The issue is that algsubs(deltaa = a1-a2, expr), my a1+a2 is replaced by deltaa+2*a2 and same for algsubs(sigmaa = a1+a2, expr), so i can't have sigmaa*ln(...) + deltaa*ln(...).

And all my squares are developed, for example I have : sqrt(1/4*a1^2+1/4*a2^2+1/2*a1*a2+...)

I can't put them in canonical form (1/2*a1+1/2*a2)^2 to get (1/2*sigmaa)^2.

Sorry for my english,
If you know how to do, i would be really grateful !

Mathieu