Maths Olympiad Prep

Library / /10 of 115

Algebra Difficulty 6.5 National olympiad Find the answer

Determine all integral solutions of a2\plusb2\plusc2\equala2b2. a^2\plus{}b^2\plus{}c^2\equal{}a^2b^2.

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

Solution

We are tasked with finding all integral solutions to the equation:

a2+b2+c2=a2b2. a^2 + b^2 + c^2 = a^2b^2.

First, let's rewrite the equation and rearrange the terms:

a2b2a2b2=c2. a^2b^2 - a^2 - b^2 = c^2.

This suggests that c2 c^2 must be non-negative, which means a2b2a2+b2 a^2b^2 \ge a^2 + b^2 .

### Case Analysis:

#### Case 1: a=0 a = 0 or b=0 b = 0

Without loss of generality, consider a=0 a = 0 . Then the equation becomes:

b2+c2=0. b^2 + c^2 = 0.

This implies that b2=0 b^2 = 0 and c2=0 c^2 = 0 , hence b=0 b = 0 and c=0 c = 0 .

Similarly, if b=0 b = 0 , we also get a=0 a = 0 and c=0 c = 0 .

Thus, one solution is (a,b,c)=(0,0,0) (a, b, c) = (0, 0, 0) .

#### Case 2: a0 a \neq 0 and b0 b \neq 0

Suppose both a a and b b are non-zero. Since a2b2a2+b2 a^2b^2 \geq a^2 + b^2 , divide both sides by positive a2b2 a^2b^2 :

1. Rearrange the equation to 1a2b2+b2a2 1 \geq \frac{a^2}{b^2} + \frac{b^2}{a^2} .

2. By the AM-GM inequality, we have:

a2b2+b2a22. \frac{a^2}{b^2} + \frac{b^2}{a^2} \geq 2.

Therefore, the equation 12 1 \geq 2 leads to a contradiction.

This contradiction implies there cannot be any non-zero integer solutions for a0 a \neq 0 and b0 b \neq 0 .

### Conclusion:

The only integral solution satisfying the equation a2+b2+c2=a2b2 a^2 + b^2 + c^2 = a^2b^2 is:

(0,0,0). \boxed{(0, 0, 0)}.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

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