What are "like terms"? Why can we only add like terms?

Like terms are terms whose variables are the same.

We can only add them because if they does not have the same variable you do not actually know if the variables are equivalent and you will get the wrong answer.

Step-by-step explanation:

Ex: 4z, 7z, 108z, 45z

Ex: 4w-2h+7a-2w

You can not add 4w and 7a because you do not know what the variables equal so you do not know the true number so add or subtract by.