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Algebra Arithmetic Functional Equations Math Olympiad PRMO

Roots and coefficients of equations | PRMO 2017 | Question 4

Try this beautiful problem from the Pre-RMO, 2017 based on Roots and coefficients of equations. You may use sequential hints to solve the problem.

Try this beautiful problem from the PRMO, 2017 based on Roots and coefficients of equations.

Roots and coefficients of equations – PRMO 2017


Let a,b be integers such that all the roots of the equation \((x^{2}+ax+20)(x^{2}+17x+b)\)=0 are negetive integers, find the smallest possible values of a+b.

  • is 107
  • is 25
  • is 840
  • cannot be determined from the given information

Key Concepts


Polynomials

Roots

Coefficients

Check the Answer


But try the problem first…

Answer: is 25.

Source
Suggested Reading

PRMO, 2017, Question 4

Polynomials by Barbeau

Try with Hints


First hint

\((x^{2}+ax+20)(x^{2}+17x+b)\)

where sum of roots \( \lt \) 0 and product \( \gt 0\) for each quadratic equation \(x^{2}\)+ax+20=0 and

\((x^{2}+17x+b)=0\)

\(a \gt 0\), \(b \gt 0\)

now using vieta’s formula on each quadratic equation \(x^{2}\)+ax+20=0 and \((x^{2}+17x+b)=0\), to get possible roots of \(x^{2}\)+ax+20=0 from product of roots equation \(20=(1 \times 20), (2 \times 10), (4 \times 5)\)

min a=4+5=9 from all sum of roots possible

Second Hint

again using vieta’s formula, to get possible roots of \((x^{2}\)+17x+b)=0 from sum of roots equation \(17=-(\alpha + \beta) \Rightarrow (\alpha,\beta)=(-1,-16),(-2,-15),\)

\((-8,-9)\)

min b=(-1)(-16)=16 from all products of roots possible

Final Step

\((a+b)_{min}=a_{min}+b_{min}\)=9+16=25.

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