Question
If a,b,c are real numbers such taht a+b+c=0, abc=2. Find the minimum value of |a|+|b|+|c|
If a,b,c are real numbers such taht a+b+c=0, abc=2. Find the minimum value of |a|+|b|+|c|
Since abc = 2>0, there must be two of them are negative and one is positive.
Let a<0, b<0 and c>0. The absolute symbols could be removed
|a|+|b|+|c|
=-a-b+c
=-(a+b)+c
Since a+b+c=0, then
On the other hand,
a+b = -c
and
ab = \dfrac{2}{c}
Apply the Vieta's formula, a,b are the two roots of the quadratic equation
x^2+cx+ \dfrac{2}{c}=0
The equaiton has real roots if and only if its discriminant is not less than 0. That is,
c^2-4\cdot \dfrac{2}{c}\geq 0
Solving the inequality gives
c\geq 2
Therefore, the minimum value of |a|+|b|+|c| is 4