The next step of your proof is to subtract (a/b) from both sides.
Then you get, x = (m/n) - (a/b)
Since rationals are closed over addition, (m/n) + (-a/b) is a rational number.
Therefore, x (an irrational number) = a rational number <em>This is a false statement which is a contradiction. So, the assumption was incorrect.</em>
Thus, the sum of a rational and irrational number is an irrational number. QED
