I'm having some difficulty understanding the meaning of "hold y (or x) constant" when finding the partial derivative of an equation such as:
$z=x^2+xy+y^2$
Please show all intermediate steps using partial derivative notation and explain what it means in practical terms to hold a variable constant using actual numbers.
Thanks!
One way of understanding it is to introduce a one-variable function and then differentiate that function in the way you already know. For instance, if you are asked to compute $\frac{\partial z}{\partial x}$ at $(x,y)=(1,2)$, you can introduce $r(x)=z(x,2)=x^2+2x+4$. Then $\frac{\partial z}{\partial x}(1,2)=r'(1)$.
If you want to get $\frac{\partial z}{\partial x}$ for general $(x,y)$, you can imagine having a different function $r$ for each value of $y$, and differentiating that function. This is not a very convenient way to do calculations, but it is a simple way of thinking about what's going on.
As a practical matter, when you compute $\frac{\partial z}{\partial x}$, you treat $y$ as if it were a number. Thus for instance $\frac{\partial}{\partial x}(xy)=y$, for the same reason that $\frac{d}{dx}(2x)=2$. Similarly $\frac{\partial}{\partial x}(y^2)=0$, for the same reason that $\frac{d}{dx}(4)=0$.