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Calculus Help!

FlyVirgin744 From United States of America, joined Jul 1999, 1313 posts, RR: 1
Posted (11 years 7 months 2 weeks 3 days 4 hours ago) and read 1839 times:

Alright all you smart people:

Regarding related rates,

Assume x and y are both differentiable functions of t and find the required values of dy/dt and dx/dt .

Equation: xy = 4

Find: dy/dt when x = 8
Given: dx/dt = 10

and

Find: dx/dt when x = 1
Given: dy/dt = -6

Please someone shed some light!

Matt

Sometimes I go about in pity for myself and all the while a great wind carries me across the sky.

4 replies: All unread, jump to last

PanAm747 From United States of America, joined Feb 2004, 4242 posts, RR: 10
Reply 1, posted (11 years 7 months 2 weeks 3 days 3 hours ago) and read 1834 times:

you have to use the multiplication rule:

xy'+yx'=4

I think, it's been a while.

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FlyVirgin744 From United States of America, joined Jul 1999, 1313 posts, RR: 1
Reply 2, posted (11 years 7 months 2 weeks 3 days 3 hours ago) and read 1831 times:

Panam, that rule does come into play but it gets quite a bit more complicated. Thank you for your response though. Smile

Sometimes I go about in pity for myself and all the while a great wind carries me across the sky.

Pgh234 From United States of America, joined Jul 2001, 790 posts, RR: 1
Reply 3, posted (11 years 7 months 2 weeks 3 days 3 hours ago) and read 1829 times:

OMG, that is SOOOOOO creepy. I am doing the EXACT SAME PROBLEM for my AP Calculus homework as I am reading your post. Do you happen to have the same book as me also? It is a red Calculus book with "Larson-Hostetler-Edwards" Second edition on the bottom of the cover and the problem you are refering to is on Page 178, problem #3a in section 2.7?

anyways, to solve:

xy = 4 dx/dt=10 and find dy/dt when X=8

(x dy/dt + y dx/dt) = 0 (Multiplication Rule as stated above)
dy/dt = -(y/x)dx/dt
dy/dt = - ((1/2)/8) * 10 (Found Y value by solving equation for X=8)
dy/dt = -5/8

Good Luck in Calculus, cuz so far, it sucks!
pgh234

FlyVirgin744 From United States of America, joined Jul 1999, 1313 posts, RR: 1
Reply 4, posted (11 years 7 months 2 weeks 3 days 2 hours ago) and read 1826 times:

Thank you so much! Seems so easy now Wink/being sarcastic

I am actually using the 6th edition of your book! Its brand new, could have some better examples though.

Well thanx again and good luck to you also in your class. And oh yes, it definetely does suck.

Sometimes I go about in pity for myself and all the while a great wind carries me across the sky.

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