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Algebra Difficulty 5.5 AIME, harder Prove it

1. (20 points) Given that aa, bb, and cc are real numbers, ac<0a c < 0, and 2a+3b+5c=0\sqrt{2} a + \sqrt{3} b + \sqrt{5} c = 0.
Prove: The quadratic equation ax2+bx+c=0a x^{2} + b x + c = 0 has a root greater than 34\frac{3}{4} and less than 1.

Solution

1. From the condition, we have 35b+c=25a\frac{\sqrt{3}}{\sqrt{5}} b+c=-\frac{\sqrt{2}}{\sqrt{5}} a. Let
y=ax2+bx+c. y=a x^{2}+b x+c .

When x=35x=\frac{\sqrt{3}}{\sqrt{5}}, we have
y1=35a+35b+c=3105a y_{1}=\frac{3}{5} a+\frac{\sqrt{3}}{\sqrt{5}} b+c=\frac{3-\sqrt{10}}{5} a \text {. }

When x=1x=1, we have
y2=a+b+c=a+b+c13(2a+3b+5c)=a3[(32)ca(53)]. \begin{array}{l} y_{2}=a+b+c=a+b+c-\frac{1}{\sqrt{3}}(\sqrt{2} a+\sqrt{3} b+\sqrt{5} c) \\ =\frac{a}{\sqrt{3}}\left[(\sqrt{3}-\sqrt{2})-\frac{c}{a}(\sqrt{5}-\sqrt{3})\right] . \end{array}

Since 310<0,53>03-\sqrt{10}<0, \sqrt{5}-\sqrt{3}>0, and ca>0-\frac{c}{a}>0, then
y1y2=31053[(32)a2ca(53)a2]>34, hence the equation has a root greater than 34 and less than 1. y_{1} y_{2}=\frac{3-\sqrt{10}}{5 \sqrt{3}}\left[(\sqrt{3}-\sqrt{2}) a^{2}-\frac{c}{a}(\sqrt{5}-\sqrt{3}) a^{2}\right]>\frac{3}{4}, \text{ hence the equation has a root greater than } \frac{3}{4} \text{ and less than 1.}

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.