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March 8th, 2017, 10:26 AM   #1
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finding dy/dt

dy/dt = x^2 + xy -y^2 and dx/dt = 8 when x = 6

Whenever I seem to try to do this problem, I can't get the y at the end to cancel and if I do it never is the correct answer.

Last edited by skipjack; March 8th, 2017 at 11:34 AM.
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March 8th, 2017, 11:06 AM   #2
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Is that the entirety of the information you were given?
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March 8th, 2017, 11:34 AM   #3
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March 8th, 2017, 11:53 AM   #4
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You are told (but omitted to tell us) that y > 0, so when x = 6, y must equal 9.
Substituting y = 9 in your answer gives dy/dt = (-96 - 72)/(6 - 18) = 14.

The above assumes the intended question is "Find dy/dt where x² + xy - y² = 9 and dx/dt = 8 when x = 6".

Last edited by skipjack; March 12th, 2017 at 06:17 AM.
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March 12th, 2017, 04:42 AM   #5
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Quote:
Originally Posted by wilkin823 View Post
dy/dt = x^2 + xy -y^2

Is this really "dy/dt" or is it the original function, y, for which you want to find the derivative?
Quote:
and dx/dt = 8 when x = 6
Quote:

Whenever I seem to try to do this problem, I can't get the y at the end to cancel and if I do it never is the correct answer.
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