units are incompatible error while adding or subtracting constants with a variable.

[MA_11053612]

I am using ptc mathcad. I am faced with a problem, i have derived an equation and when I plug in the values there is a constant adding with a variable like (1+a1) or (1-a1) the software shows an error that the units are not compatible. Well if the equation solved the unit in the nominator and denominatio cancel each other. What can i do to fix the issue. 
its like it considers constants to have a unit

[ChrisKaswer · Accepted solution]

This is a straight-forward issue - you can't subtract a distance value from a number. Your "1" needs to have units of length associated with it to be compatible in your relation where you subtract a length from "just a number".