Skip to main content

2002 AMC 12B Problem 11

Problem 11 of 25IntermediateNumber Theory

The positive integers A,A, B,B, A−B,A-B, and A+BA+B are all prime numbers. The sum of these four primes is

Answer choices

Show solution

Solution

A−BA-B and A+BA+B have the same parity; being prime, both are odd, so AA and BB have opposite parity. If AA were even, then the prime AA would equal 2,2, but the positive prime BB would satisfy B≥2B\ge2 and make A−B≤0.A-B\le0. Hence AA is odd and the even prime BB is 2.2. Then A−2,A-2, A,A, A+2A+2 are three primes. One is divisible by 3,3, so that one must equal 3;3; the triple is 3,3, 5,5, 7.7. Their sum together with 22 is 2+3+5+7=17,2+3+5+7=17, a prime. Thus, the correct answer is E.
AoPS wiki

Tagged: parity · prime

More practice