1, you can substitute the solution into the equations: m= 1/2 n= 1/2.

2.( 1) No solution

(2) There are countless solutions. Equation (1) is enlarged by two times, which is equation (2). Only the solution satisfying the equation is taken.

(3) No solution

3. If a=2 is substituted into the equation, 2x+y= 1 and 2x+y=2 are contradictory, that is, there is no solution.