Solving Addition and Subtraction Equations

An equation is a

mathematical sentence

that contains an equal sign.

The expression on the left and right side of the equal

sign are equivalent. One or

both sides may contain a variable.

A variable is a letter or

symbol that stands for a

number.

To solve an equation, you

must find what the variable

represents so that the two

parts are equal.

Equations are like balance scales. In order to keep a scale balanced you must have the same amount of weight on both sides.

If I took one block away from the left side, what would I have to do to keep

the scale from tipping?

If I added 6 more apples to the right side, how could I keep the scale from

tipping?

To keep an equation equal, we must do exactly the same

thing to each side of the equation. If we

add a number to one side, we must add

that same number to the other side and

vice versa.

Addition and subtraction are called “inverse operations”.

Multiplication and

division are inverse

operations, too!

for Solving Addition and Subtraction Equations

1. Look to see what side the variable

is on.

2. Decide what

operation is being shown.

3. Do the inverse of

that operation to both sides.

4. Write what

number the variable

stands for (ex: a = 2).

5. Check the solution by “substituting” the number back into the equation to see if the sides are equivalent.

Example 1 (bar model) m + 18 = 34 Look for variable

Do the inverse What number does m represent?

m = 16

Substitute in m

16 + 18 = 34

34 = 34

Solution is correct!

m 18

34

16 18

34

Example 1m + 18 = 34 Look for variable

-18 -18 Do the inverse

m = 16 Write what m is

m + 18 = 34

16 + 18 = 34 substitute in m

34 = 34 Solution is correct!

Always remember to check your solution to the

equation.

Example 2 68 = y + 43 Look for variable

-43 -43 Do the opposite

25 = y Write what y is

68 = y + 43

68 = 25 + 43 Plug in y

68 = 68 Solution is correct!

Example 3 t - 14 = 22 Look for variable

+ 14 +14 Do the inverse

t = 36 Write what t is

t - 14 = 22

36 - 14 = 22 Substitute in t

22 = 22 Solution is correct!

Try These...1. r + 5 = 18

2. m - 26 = 59

3. 102 = x - 15

4. a + 39 = 56

r = 13

m = 85

117 = x

a = 17

Prediction:You will show all

of the steps (including

checking) as you have been shown

today

Are there any questions

about solving addition or subtraction equations?