How to calculate marginal utility with two goods? I'm just getting started as an amateur into microeconomics and I really can't understand this thing about marginal utility when more than one good is involved:

Let's say I have the utility function U = x(y+1). 

Now, from what I've studied, I  think that the marginal utilities for x and y should be:

MUx = y+1
MUy = x

I just don't get how to arrive there "mathematically", and I fear I wouldn't be able to find the marginal utilities of a more complex function, for example U = x(x+y) 

Could you help me?
Thank you!
This is quite simple to answer given you know a bit of multivariable differential calculus. You're looking for partial derivatives of the utility function. 

So, given $$U(x,y) = x(y+1)$$ we have $$\frac{\partial U}{\partial x} = y+1$$ and $$\frac{\partial U}{\partial y} = x$$. 

These are the goods' marginal utilities.