What is the electric potential derivative when x=2 and y=1?

[IslandHead]

Homework Statement

The electrical potential can be described by the following equation:
V= 200/(x² + y² )^1/2 find dV when x=2 and y=1

Homework Equations

n/a

The Attempt at a Solution

dv/d(x,y) = ∂/∂x + ∂/∂y
=200/(x² + y²)^(1/2) +200/(x² + y²)^(1/2)
replace variables with C where necessary and rearrange for easy of deriving =200(x² + C)^(1/2) +200(C + y²)^(1/2)
Chain rule and exponent rule
=200(-1/2)(2x)(x² + C)^(-3/2) +200(-1/2)(2y)(C + y²)^(-3/2)
=-200x(x² + C)^(-3/2) + -200y)(C + y²)^(-3/2)
=-200x(x² + y²) )^(-3/2) + -200y)(x²+ y²)^(-3/2)
Then I plug in the values x=2 and y=1 and dV=-53.7 V/m, I'm not very confident with multivariable calculus and just wanted to check to see if this is right.

 

[gneill]

The result looks okay.