Help me please with a demonstration 'reductio ad absurdum'!

1. - substitution

- be a and b,two numbers greater or equal than 1 from the set of real numbers R,

- be n a number greater or equal than 3 from the set of real numbers R,

2. - conclusion

- there is always a and b such that equation is true: n=a+b+1

3. - demonstration

- ???

Thank you very much your all helps !