Lets establish something first..
A is not a cell reference, A1 is a cell reference.
So assuming the following.
A1=500
B1=750
C1=750
Cells can contain a value or a formula, not both.
Since B1 and C1 already contain a value you cant put a formula in them as well.
So you have to use other cells
in D1
=B1+A1/2
in E1
=C1+a1/2
Now use D1 and E1 instead of B1 and C1