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.
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( a + b + c ) 2 = 1 0 0 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
⇒ ( c + b ) ( a + c ) = 2 5 ∗ 5 6
The only factoring possible is a + c = 6 2 5 , b + c = 8 0 0 ⇒ b − a = 1 7 5 ⇒ a 2 + ( a + 1 7 5 ) 2 = ( 1 0 0 0 − 1 7 5 − 2 a ) 2 ⇒ a = 2 0 0 , b = 3 7 5 , c = 4 2 5
⇒ a b c = 3 1 8 7 5 0 0 0
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We have a Pythagorean triplet: 8, 15, 17 where 8 + 1 5 + 1 7 = 4 0 .
We know that if a 2 + b 2 = c 2 , then ( a k ) 2 + ( b k ) 2 = ( c k ) 2
Since 4 0 × 2 5 = 1 0 0 0 , then ( 8 × 2 5 ) 2 + ( 1 5 × 2 5 ) 2 = ( 1 7 × 2 5 ) 2 and ( 8 + 1 5 + 1 7 ) × 2 5 = 1 0 0 0
So a b c = 8 × 1 5 × 1 7 × 2 5 3 = 3 1 8 7 5 0 0 0