Calculus-differentiation Help Please

calculus--differentiation Help Please!!!

if [tex]x=\rho cos \theta[/tex] and [tex] y=\rho sin \theta[/tex]

prove that if U is a twice differentiable function of x and y that
[tex]\frac{\partial^2U}{\partial x^2} + \frac{\partial^2 U}{\partial y^2} = \frac{\partial^2 U}{\partial \rho^2} + \frac{1}{\rho}\frac{\partial U}{\partial \rho} + \frac{1}{\rho^2}\frac{\partial^2 U}{\partial \theta^2}[/tex]

I have absolutely no clue how to get started on this one.

thanks

 

anyone...........

 

I could try this except for the fact that i do not know what U equals...is there some assumption i am supposed to make here

 

i'm sorry i don't quite understand what i am supposed to be doing here. can i get a some explanation as to how and why to start this. i have really no idea here. thanks