To subtract a monomial from another of the same variables just subtract the coefficients. Only two like monomials can be subtracted from each other.
By subtracting 3x from 4x, we get (4-3)x=x.
By subtracting 3xy from 2xy, we get (2-3)xy=-xy.
By subtracting bx from ax, we get ax-bx=(a-b)x.
Monomials can be subtracted only if they have the same variables with identical powers.
GRE考试数学考点：Combinations of Choices
Combinations of Choices
If there are n choices for one type and m choices for second type, then number of combinations for both types is the product of choices:n*m.
If there are 3 kinds of breads to choose from and 4 kinds of meats, then we can make 3*4=12 different sandwiches.
If an alphabet has 26 letters, then the number of possible 2 letter word is 26*26=676.All of them may not be vaild words.
Do not add the choices to get the total combinations.
The product rule can be applied to more than two types of choices: if there are 3 kinds of breads, 4 kinds of meats, and 6 kinds of cheeses, then we can make 3*4*6=72 different sandwiches.