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 If x + y = sqrt(31) and x - y = sqrt(21), then find xy.

 May 24, 2020
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add the two equations to get    2x = sqrt31 + sqrt21      x = 1/2 ( sqrt31+sqrt21)

multiply second  equation by -1 to get    -x+y = -sqrt21    add to first equation to get

     2 y = sqrt31 - sqrt21

         y = 1/2 (sqrt31 - sqrt21)

               multiply the red equations together       31/4    -  21/4 = 10/4 = 2.5

 May 24, 2020

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