My Math Forum Finding the rate of change at a point in the direction of a vector

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 February 17th, 2015, 09:29 PM #1 Member   Joined: Jan 2015 From: Colorado Posts: 32 Thanks: 0 Finding the rate of change at a point in the direction of a vector Hi, I am confused on how to find the rate of change at a point in the direction of a vector, I know that when you are finding the rate of change at a point to another point, you take the change in distance for those two points, but I am not sure how to do that for this problem... What is the rate of change of f(x,y)=2xy+y2 at the point (3,2) in the direction v=−i+3j?
 February 18th, 2015, 04:49 AM #2 Math Team   Joined: Dec 2013 From: Colombia Posts: 7,551 Thanks: 2554 Math Focus: Mainly analysis and algebra This is a directional derivative. We are surfing along a line looking at how $f(x,y)$ changes along that line. In this case our line is $<3,2>+t<-1,3> = <3-t, 2+3t>$ where $t$ varies as we move along the line. And at $t=0$ we are at $<3,2>$. Thus we have the directional derivative$$f'(3,2) = \lim_{t \to 0} {f(3-t,2+3t) - f(3,2) \over t}$$ This formula leads to $${\mathrm d \over \mathrm d t}f(x,y) = {\partial f \over \partial x}{\mathrm dx \over \mathrm d t} + {\partial f \over \partial x}{\mathrm dx \over \mathrm d t}$$ In our case $x = 3-t$ and $y=2+3t$.

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