Question

Find the directional derivative:
of f(x,y) = ye^(-x) at point (2,1), theta = pi/4

so I got
u = (cos(pi4), sin(pi/4))
f_x(x,y) = -ye^-x
f_y(x,y)= e^-x

thus I got

Duf(2,1) = (-1/e^2)(cos(pi/4)), (1/e^2)(sin(pi/4)))

is this correct?

Answer 1

looks like i'm wrong :L

Answer 2

I'm suppose to add them I think
(-1/e^2)(cos(pi/4))+ (1/e^2)(sin(pi/4)))

Answer 3

I guess not I get zero if I add them, and the answer is \[2 + \sqrt{3}/2\]

Answer 4

oh crud used the wrong coordinates, should be 0,4

Answer 5

I was already wondering where the mistake could be, what you did is correct isn't it?

Answer 6

let me rework out the problem I used the wrong point

Answer 7

Wow brain isn't working:
Duf(0,4) = -4(cos(pi/4)) + 1(sin(pi/4)))

Answer 8

I still don't get the right answer that equals to -3/(2)^(1/2)

Answer 9

I used the wrong theta as well lol

Answer 10

Duf(0,4) = -4(cos(2pi/3)) + 1(sin(2pi/3)))

Answer 11

ok never mind I think I got it

Answer 12

I remember the good old times where I was 'stuck' at a problem, staring at a solution and unable to derive it, 30-60 minutes later I noticed that I was looking at the solution for a different exercise.

Answer 13

lol

Answer 14

good times, good times. Long evenings (-; well glad you managed it!