Olympiad Maths Prep

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Problem 245

AMC 10/12, early questions
Algebra Difficulty 3.9 Prove it

If aa, bb, and cc are rational numbers and the equation a+b32+c34=0a + b\sqrt{32} + c\sqrt{34} = 0 holds, prove that a=b=c=0a=b=c=0.

This one wants a proof. Work it on paper, read the official solution, then mark yourself honestly — the ladder only means something if the record is true.

Official solution

Let's suppose that at least one of aa, bb, or cc is not equal to 00. We will check each assumption to see if the equation holds:

1. If a=b=0a=b=0 and c0c\neq 0, the equation becomes c34=0c\sqrt{34} = 0. Since 34\sqrt{34} is not 00, cc must be 00 to satisfy the equation, which is contradictory to the assumption.

2. If a0a\neq 0 and b=0b=0, the equation becomes a+c34=0a + c\sqrt{34} = 0. This implies c34=ac\sqrt{34} = -a. Since aa and cc are rational numbers, and the square root of a non-square integer is an irrational number, 34\sqrt{34} is irrational. Therefore, the product of cc (a rational number) and 34\sqrt{34} (an irrational number) cannot equal aa (a rational number). This leads to a contradiction.

In both cases, we arrive at a contradiction, and therefore, aa, bb, and cc must all be equal to 00 to satisfy the equation.

a=b=c=0 \boxed{a = b = c = 0}

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