Question: How to get simple answer when using assuming?

This is my code
 

restart;
with(Student:-MultivariateCalculus);
A := [0, 0, 0];
B := [a, 0, 0];
C := [a, a, 0];
DD := [0, a, 0];
S := [0, 0, h];
d1 := Line(A, C);
d2 := Line(S, DD);
(Distance(d1, d2) assuming (a::positive and h::positive));

I got 

abs(a)^2*abs(h)/sqrt(2*abs(a*h)^2 + abs(a)^4)

How can I remove abs?

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