You add or subtract only the numerators
Only like terms can be subtracted or added in algebraic expressions.
Adding like terms can be like adding fractions. You can only add fractions with a common denomonator. You can only combine terms together if they are like. Think of like terms as denomonators. You can only add if they are like.
To add and subtract algebraic expressions the simple rule of like terms applies. In your homework that asks for the expression represents the perimeter in units of this trapezoid you will need to find the like terms and simplify.
Numerators
You add or subtract only the numerators
Only like terms can be subtracted or added in algebraic expressions.
You can only add or subtract like terms. So, you subtract 9x from 2x, which gives you -7x. Then subtract -9 from 3 to get -6. And you are left with -7x -6.
You add (or subtract) like terms. This will reduce the number of terms in the expression and that is the extent of simplification that you can achieve using this process.
Because addition and subtraction are defined in terms of like terms.
Adding like terms can be like adding fractions. You can only add fractions with a common denomonator. You can only combine terms together if they are like. Think of like terms as denomonators. You can only add if they are like.
To add and subtract algebraic expressions the simple rule of like terms applies. In your homework that asks for the expression represents the perimeter in units of this trapezoid you will need to find the like terms and simplify.
Numerators
You can add and subtract any like terms. Since 8w, -4w, and 4w are all like terms, since 8, -4, and 4 all tell how many w's you have. This is the essentially same as saying "There are 8 apples, subtract 4 apples, and add 4 apples." (Don't forget that 8w-4w is the same as 8w + -4w). To solve, just add 8, subtract 4, and add 4. Leave w alone. The answer is 4w.
You can add and subtract coefficients when they occur in like terms, which means they have the same variable. 2x + 3x = 5x (This addition can take place because all of the variables are x, and are therefore like terms.) and 5y - 2y = 3y (This subtraction can take place because all of the variables are y, and are therefore like terms.) However, if the variables differ, they are unlike terms, and you cannot add or subtract the coefficients. For example: 2a + 3b cannot be added because a and b are different variables, and therefore unlike terms. 3c - 2d cannot occur because c and d are different variables, and therefore unlike terms..
Terms like 2x and 4x2 are not like terms. In order for terms to be like terms, they must have the exact same variables with the exact same exponents on them. This means that 5x3 and 7x3 are like terms, but 2x and 4x2 are not. To add or subtract like terms, simply add or subtract the coefficients (the numbers in front) and keep the variable part the same. For example, 5x3 + 7x3 = (5 + 7)x3 = 12x3, and 3x2 + 6x2 = (3 + 6)x2 = 9x2. But, since 2x and 4x2 are not like terms, 2x + 4x2 = 2x + 4x2. It just cannot be simplified any more than that.
Terms with the same variable(s). In like terms, you may add, subtract, multiply, or divide them. In the equation 3x - 5ax + 9x = 7ax, the 5x and the 10x are like terms because they both contain variables of the same value. Thus, we are allowed to add them up and get 12x. The -5ax and the 7ax are also like terms and if we add 5ax to both sides (after we add 3x to 9x), we'll get the equation 12x = 12ax.