Consider the equation , in which a , b and c are real numbers and . Suppose that this equation has two real roots, r and s, where and . Is it true to say that ?
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As it is known and proven, the sum of a quadratic equation's roots can be expressed as a − b , and their product, as a c . Hence:
r + s = a − b and r × s = a c
So r = a − b − s ⇒ r = a − b − a . s . As r > 0 :
a − b − a . s > 0 ⇒ − b − a . s > 0 ⇒ − b > a . s ⇒ b < − a . s
As a > 0 and s > 0 , "-a.s" is negative, so "b" is a negative number. Repeating this process with r × s :
r = a . s c ⇒ a . s c > 0
As a . s > 0 (because a > 0 ), "c" is a positive number.
Therefore, b + c < 0 is true only if − b > c or ∣ b ∣ > ∣ c ∣ . As r and s are decimal numbers, where 0 < r < 1 and 0 < s < 1 , their sum is always greater than their product. So:
r + s > r . s ⇒ a − b > a c ⇒ − b > c ∴ b + c < 0