1       1
--- x + --- y
 2       2

The screens below show the following points as an example. Of course, one example does not prove that the formula always works but it is one rough check that might detect some errors.

a = (1,1)
b = (5, 3)

 1       1
--- a + --- b
 2       2

using the TI-92 instructions

[1, 1] -> a
[5, 3] -> b
Showpts({"a", "b", "(1/2)*a + (1/2)*b"}, 0,11,0,5)