The Complexity's Simplicity

Algebra Level 3

Consider the equation a x 2 + b x + c = 0 ax^{2} + bx + c = 0 , in which a , b and c are real numbers and a > 0 a > 0 . Suppose that this equation has two real roots, r and s, where 0 < r < 1 0 < r < 1 and 0 < s < 1 0 < s < 1 . Is it true to say that b + c < 0 b + c < 0 ?

This problem was not created by me. It is from the Brazilian Maths Olympiads (OBM), of the year 2013.
Not always, but not never. No. Yes. Cannot be determined.

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1 solution

As it is known and proven, the sum of a quadratic equation's roots can be expressed as b a \frac{-b}{a} , and their product, as c a \frac{c}{a} . Hence:

r + s = b a r + s = \frac{-b}{a} and r × s = c a r \times s = \frac{c}{a}

So r = b a s r = b a . s a r = \frac{-b}{a} - s \Rightarrow r = \frac{-b -a.s}{a} . As r > 0 r > 0 :

b a . s a > 0 b a . s > 0 b > a . s b < a . s \frac{-b -a.s}{a} > 0 \Rightarrow -b - a.s > 0 \Rightarrow -b > a.s \Rightarrow b < -a.s

As a > 0 a > 0 and s > 0 s > 0 , "-a.s" is negative, so "b" is a negative number. Repeating this process with r × s r \times s :

r = c a . s c a . s > 0 r = \frac{c}{a.s} \Rightarrow \frac{c}{a.s} > 0

As a . s > 0 a.s > 0 (because a > 0 a > 0 ), "c" is a positive number.

Therefore, b + c < 0 b + c < 0 is true only if b > c -b > c or b > c \lvert b \rvert > \lvert c \rvert . As r and s are decimal numbers, where 0 < r < 1 0 < r < 1 and 0 < s < 1 0 < s < 1 , their sum is always greater than their product. So:

r + s > r . s b a > c a b > c b + c < 0 r + s > r.s \Rightarrow \frac{-b}{a} > \frac{c}{a} \Rightarrow -b > c\therefore \LARGE \boxed{b + c < 0}

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