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If they exist, find two numbers whose difference is 100 and whose product is a minimum....

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cspanutius | Student, Undergraduate | (Level 1) Salutatorian

Posted July 24, 2012 at 10:55 PM via web

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If they exist, find two numbers whose difference is 100 and whose product is a minimum. If such two numbers do not exist, explain why.

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beckden | High School Teacher | (Level 1) Educator

Posted July 25, 2012 at 1:28 AM (Answer #1)

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x - y = 100 and xy is a minimum.

1st we solve `x-y=100` for y to get `y = x - 100` . substituting we get `xy = x(x-100) = x^2 - 100x`

`f(x) = x^2 - 100x` ,  `f'(x) = 2x - 100` ,  this is zero when x = 50, so y = -50.  We can use the second derivative test to see `f''(x) = 2` which is positive, so (50, -50) is a minimum.  

Answer:  50, -50 are the two numbers. 

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