You do not have to figure out what type of math or substitution each equation is, you just have to understand the basic how of the way each one comes to its answer and how they relate to each other. If you come up with a thought, try it and see if it works, don't just assume "it can't be that."
Hint: Notice how everything is similar to each other. Look at how the variables are being added to become the sum. Try it!