Maths Olympiad Prep

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Algebra Difficulty 5.5 AIME, harder Find the answer

Example 1 Given the proposition pp: The equation a2x2+ax2=0a^{2} x^{2}+a x-2=0 has a solution in [1,1][-1,1]; proposition qq: There is only one real number xx that satisfies the inequality x2+2ax+2a0x^{2}+2 a x+2 a \leqslant 0. If the proposition “pp or qq” is false, find the range of values for aa.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Solve: From a2x2+ax2=0a^{2} x^{2}+a x-2=0 we get (ax+2)(ax1)=0(a x+2)(a x-1)=0.
Obviously a0,x=2aa \neq 0, \therefore x=-\frac{2}{a} or x=1ax=\frac{1}{a}.
Since x[1,1]x \in[-1,1], thus 2a1\left|\frac{2}{a}\right| \leqslant 1 or 1a1\left|\frac{1}{a}\right| \leqslant 1, so a1|a| \geqslant 1.
There is only one real number satisfying x2+2ax+2a0x^{2}+2 a x+2 a \leqslant 0, which means the parabola y=x2+2ax+2ay=x^{2}+2 a x+2 a intersects the xx-axis at only one point, so Δ=4a28a=0\Delta=4 a^{2}-8 a=0, thus a=0a=0 or 2.
The proposition “pp or qq” is true when a1|a| \geqslant 1 or a=0a=0.
Since the proposition “pp or qq” is false, the range of values for aa is {a1<a<0\{a \mid-1<a<0 or 0<a<1}0<a<1\}.
Comment: This problem can also be solved by first using a truth table to determine that both pp and qq are false propositions, and then solving the problem.

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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.