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Quadratic inequality leading to sum of roots (Posted on 2021-04-02) Difficulty: 2 of 5
Let P(x) be a real quadratic trinomial, so that for all x∈R the inequality P(x3+x)≥P(x2+1) holds. Find the sum of the roots of P(x).

No Solution Yet Submitted by Danish Ahmed Khan    
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Solution Solution | Comment 1 of 2
The sum of the roots of the quadratic P(x)=ax^2+bx+c is -b/a

If we expand the sides of the inequality as subtract the terms from the right we get 
ax^6+ax^4+bx^3+(-a-b)x^2+bx0
x(ax^5+ax^3+bx^2+(-a-b)x+b)0

For an inequality of the form pq0 to hold it must be true that if p>0, q>0 and if p<0, q<0 and if p=0, q=0.  In other words

ax^5+ax^3+bx^2+(-a-b)x+b must equal 0 if x=0

this means b=0, so the sum of the roots of P = -b/a=0

  Posted by Jer on 2021-04-02 10:18:00
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