# multivariable calculus – What does it mean to take F(x, y) as F(x, y(x))?

What does it mean to take $$F(x, y)$$ as $$F(x, y(x))$$? I just do, not, get the difference, despite numerous explanations.

To put it into context, I see this done when applying the chain rule to $$F$$ to find $$frac{mathrm{d}F}{mathrm{d}x}$$.

So what does it mean? What is the difference between $$F(x, y)$$ and $$F(x, y(x))$$?

i.e. take $$F(x, y) = x^2 + y^3 + 2y$$. What would $$F(x, y(x))$$ be equal to?

Please dumb explanations down, and don’t skip steps, as I’ve heard numerous explanations and don’t get any of them!