Two positive numbers have the property that the sum of their squares is 30 and the sum of the reciprocals is 2 what is their product?
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Since x 1 + y 1 = 2 ⟹ x y x + y = 2 ⟹ x + y = 2 x y .
Since x 2 + y 2 = 3 0 ⟹ x 2 + 2 x y + y 2 = 3 0 + 2 x y ⟹ ( x + y ) 2 = 3 0 + 2 x y .
Thus, we have ( 2 x y ) 2 = 3 0 + 2 x y .
Substitute w = x y , then 4 w 2 − 2 w − 3 0 = 0 ⟹ 2 ( w − 3 ) ( 2 w + 5 ) = 0 .
For positive numbers, we have x y = w = 3 .