I don't know of a simple way to do this, but here's a complicated way:
Split the string "B+sqrt(V ²((X-A) ²/H ²))" up into all the parts between the variables, like so:
B , "+sqrt(" , V , "²((X-" , A , ")²/" , H , "²))",
Where the commas separate the variables.
So, now you can use the "number to string" routine provided on this website, or you an make your own. In any event, convert each of the variables to a string, and compile the composite string in the correct order. Now, when you store it to Y1, the values should be displayed rather than the variables.
:Disp "H,V -Axis Radius?"
:Disp "Center? X,Y"
:LinReg(ax+b) Y₁ //Visit the number to string routine on this site to understand why this is used
It seems strange that I can't find a simpler way to do this. I hope someone else has a better solution on hand.