# How to graph gradient vector?

I'm working on a practice problem for my Calculus 3 course. It gives the function: $z=x^2+y^2$, and asks to graph the contours for $c=1,2,3$. Than asks to calculate the gradient at point $(2,1)$ and graph the result.

I'm fine with the first part. I'm letting z=c and solving for y, then graphing the result.It's the gradient portion I'm having issues with.

The gradient I came up with is: $\nabla(x^2+y^2)=\langle 2x,2y\rangle$, at $(2,1)$, = (4,2)

I'm not exactly sure how to graph this. Am I supposed to graph a line from $(2,1)$ to $(4,2)$ or what? Any help/explanation would be very much appreciated!

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I would draw the vector $(4,2)$ starting at the point $(2,1)$ (ie, a line between $(2,1)$ and $(6,3)$). – copper.hat Mar 4 '13 at 6:53
How did you get (6,3)? – PseudoPsyche Mar 4 '13 at 6:53
No, a line connecting $(2,1)$ to $(6,3) = (2,1) + (4,2)$. – copper.hat Mar 4 '13 at 6:54
For the contours you just let $z=c$ and solve for $y$ and graph the resulting function – MITjanitor Mar 4 '13 at 6:54
@copper.hat, thank you! – PseudoPsyche Mar 4 '13 at 7:02

Gradients are drawn from the point that they're taken at. This shows where gradients are taken from, and allows gradients to be perpendicular to level curves. Since the gradient was taken at the point $(2,1)$, the vector $\langle 4,2 \rangle$ should be drawn from $(2,1)$ pointing to the point $(6,3)$ because $(2,1) + (4,2) = (6,3)$.