Let a,b,c be real numbers with a ≠ 0. Suppose a and 4a + 3b + 2c have the same sign. Then
equation ax2 + bx + c = 0 cannot have both the roots in the interval.
(1,2)
Note that
If both ∝ and β lie in (1,2), then each of the terms in the right hand side is strictly negative. But this contradicts (1). Hence the given equation cannot have both roots in the interval (1,2).