Special Pythagorean triplet

A Pythagorean triplet is a set of three natural numbers, a < b < c, for which,

a² + b² = c²

For example, 3² + 4² = 9 + 16 = 25 = 5².

There exists exactly one Pythagorean triplet for which a + b + c = 1000.

Find the product abc.

The problem is not original.


The answer is 31875000.

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2 solutions

Jihoon Kang
Jul 22, 2015

We have a Pythagorean triplet: 8, 15, 17 where 8 + 15 + 17 = 40 8+15+17=40 .

We know that if a 2 + b 2 = c 2 a^2+b^2=c^2 , then ( a k ) 2 + ( b k ) 2 = ( c k ) 2 (ak)^2+(bk)^2=(ck)^2

Since 40 × 25 = 1000 40 \times 25=1000 , then ( 8 × 25 ) 2 + ( 15 × 25 ) 2 = ( 17 × 25 ) 2 (8 \times 25)^2+(15 \times 25)^2=(17 \times 25)^2 and ( 8 + 15 + 17 ) × 25 = 1000 (8+15+17) \times 25 = 1000

So a b c = 8 × 15 × 17 × 2 5 3 = 31875000 abc=8 \times 15 \times 17 \times 25^3=31875000

Maria Kozlowska
Sep 10, 2015

( a + b + c ) 2 = 100 0 2 a 2 + b 2 + c 2 + 2 a b + 2 a c + 2 b c = 2 6 5 6 c 2 + a b + a c + b c = 2 5 5 6 (a+b+c)^2=1000^2 \Rightarrow a^2+b^2+c^2+2ab+2ac+2bc=2^6 *5^6 \Rightarrow c^2+ab+ac+bc=2^5 *5^6

( c + b ) ( a + c ) = 2 5 5 6 \Rightarrow (c+b)(a+c)=2^5 *5^6

The only factoring possible is a + c = 625 , b + c = 800 a+c=625, b+c=800 b a = 175 a 2 + ( a + 175 ) 2 = ( 1000 175 2 a ) 2 a = 200 , b = 375 , c = 425 \Rightarrow b-a=175 \Rightarrow a^2 + (a+175)^2=(1000-175-2a)^2 \Rightarrow a=200, b=375, c=425

a b c = 31875000 \Rightarrow abc=31875000

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