Yahoo Answers is shutting down on May 4th, 2021 (Eastern Time) and the Yahoo Answers website is now in read-only mode. There will be no changes to other Yahoo properties or services, or your Yahoo account. You can find more information about the Yahoo Answers shutdown and how to download your data on this help page.

? asked in Science & MathematicsMathematics · 9 years ago

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

Relevance
  • 9 years ago
    Favorite 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
Still have questions? Get your answers by asking now.