The equation is x 2+(m+1) x+m+n+1= 0. This equation has two positive real roots, one is greater than 1 and the other is less than 1. Then use Vieta theorem to make m+ 1 <-1, and then make the product of the two roots on the left side of the equation f (x) and f (x+ 1) = 0 less than 0. So that you can get the answer. Do the math yourself.