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Algebra Difficulty 3.8 AMC 10/12 Find the answer United States

Problem:

If a@b=a3b3aba @ b = \frac{a^{3} - b^{3}}{a - b}, for how many real values of aa does a@1=0a @ 1 = 0?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Solution:

If a31a1=0\frac{a^{3} - 1}{a - 1} = 0, then a31=0a^{3} - 1 = 0, or (a1)(a2+a+1)=0(a - 1)(a^{2} + a + 1) = 0.

Thus a=1a = 1, which is an extraneous solution since that makes the denominator of the original expression 00, or aa is a root of a2+a+1a^{2} + a + 1.

But this quadratic has no real roots, in particular its roots are 1±32\frac{-1 \pm \sqrt{-3}}{2}.

Therefore there are no such real values of aa, so the answer is 0\mathbf{0}.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.