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Competition math problem?
Two positive numbers have the property that the sum of their squares is 20 and
the sum of their reciprocals is 2. What is their product? Express your answer
as a common fraction.
I know the answer but could you show me how to get there.
2 Answers
- Light YagamiLv 59 years agoFavorite Answer
let the numbers be a and b
a^2 + b^2 = 20 => (a+b)^2 - 2ab = 20 ->equation 1
1/a + 1/b = 2 => (a+b)/ab = 2 => a+b = 2ab
substituting this in equation 1,
4(ab)^2 - 2ab -20 =0
=> 2(ab)^2 - ab - 10 = 0
=> 2(ab)^2 - 5ab + 4ab - 10 =0
=> ab(2ab - 5) + 2(2ab -5) =0
=> (ab+2) (2ab-5) = 0
=> ab = -2 which is not possible since both numbers are positive,
or ab = 5/2 which is possible...
hence answer: product is 5/2
- 9 years ago
let the numbers be x and y
x² + y² = 20 --------- 1
1/x + 1/y = 2 --------- 2
now rearrange eq. 2
( x + y ) / (xy) = 2
x + y = 2xy
now square the eq
x² + y² + 2xy = 4x²y²
20 + 2xy = 4x²y²
let the product xy be 'm'
2m² = 10 + m
2m² - m - 10 = 0
m= -2 , 5/2
the product obvs cant be -2 because x and y are positive
therefore, their product is 5/2
Source(s): Math lover :D