Originally Posted by

**bobred** We're asked is it valid and if so give a deduction by modus ponens and proof by contradiction.

(i)

**We know that:**

If the equation is quadratic, then Dave can solve it;

Dave cannot solve the equation f(x)=0.

**We conclude that: **

The equation f(x) = 0 is not quadratic.

(ii)

**We know that:**

If the equation is quadratic, then Dave can solve it;

Dave can solve the equation f(x)=0.

**We conclude that: **

The equation f(x) = 0 is quadratic.

This is not correct. You cannot make any conclusion here.