GRE考试数学考点：Adding Monomials
To add two monomials of the same variables just add the coefficients.
Only two like monomials can be added together.
Example:
By adding 3x and 4x, we get (3+4)x=7x.
By adding 3xy+2xy, we get (3+2)xy=5xy.
By adding as and bx, we get ax+bx=(a+b)x.
Remember:
Monomials can be added only if they have the same variables with identical powers.
GRE考试数学考点：Algebraic Expression
One or many terms connected by algebraic operations is called an algebraic expression. The operations between the terms must be valid to yield an expression.
Example
(3x+5),(ax2+c),and (xy+a)(x+y) are all algebraic expressions.
(2x+5)+·(3x) is not an expression since + followed by· is not a valid sequence of operations.
Remember
There is no=sign in an expression.
