The product of two positive numbers is 6 1 6 . If the ratio of the difference of their cubes to the cube of their difference is 1 5 7 : 3 , what is the sum of the two numbers?
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Let the numbers be a and b . Then a b = 6 1 6
( a − b ) 3 a 3 − b 3 = 3 1 5 7
Solving, we get a 2 + b 2 = 1 2 6 8 ⟹ ( a + b ) 2 = a 2 + b 2 + 2 a b = 1 2 6 8 + 1 2 3 2 = 2 5 0 0 ⟹ a + b = 5 0 .
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Let the two positive numbers be a and b , such that a > b . Then a b = 6 1 6 and
( a − b ) 3 a 3 − b 3 3 ( a 3 − b 3 ) 3 ( a − b ) ( a 2 + a b + b 2 ) 3 ( a 2 + a b + b 2 ) 3 ( a 2 + 2 a b + b 2 ) − 3 a b 3 ( a + b ) 2 − 3 a b 1 5 4 ( a + b ) 2 ( a + b ) 2 ⟹ a + b = 3 1 5 7 = 1 5 7 ( a − b ) 3 = 1 5 7 ( a − b ) 3 = 1 5 7 ( a 2 − 2 a b + b 2 ) = 1 5 7 ( a 2 + 2 a b + b 2 ) − 6 2 8 a b = 1 5 7 ( a + b ) 2 − 6 2 8 a b = 6 2 5 a b = 1 5 4 6 2 5 ⋅ 6 1 6 = 2 5 0 0 = 5 0