Respuesta :
The "simple answer" is incorrect. (Actually, it's hard to tell, since the signs in your expression are missing.)
If c is positive in x^2 +bx +c, then the signs of m and n must be the same, and both must match the sign of b.
If c is positive in x^2 +bx +c, then the signs of m and n must be the same, and both must match the sign of b.