Solving Equations with Inverse Operations

Transcription

1 Solving Equations with Inverse Operations Math 97 Supplement LEARNING OBJECTIVES 1. Solve equations by using inverse operations, including squares, square roots, cubes, and cube roots. The Definition of Inverse Operations A pair of inverse operations is defined as two operations that will be performed on a number or variable, that always results in the original number or variable. Another way to think of this is that the two inverse operations undo each other. For example, addition and subtraction are inverse operations since we can say x x. If we start with x, then add and subtract, we are left with the original starting variable x. There are several inverse operations you should be familiar with: addition and subtraction, multiplication and division, squares and square roots (for positive numbers), as well as cubes and cube roots. The following examples summarize how to undo these operations using their inverses. Using Inverse Operations with the 4 Basic Operations Addition Subtraction Multiplication Division Solve: x. Solve: x. Solve: x 8. x Solve: 8. x has added to it, so we subtract from x has subtracted from it, so we add to x has multiplied to it, so we divide from x is divided by, so we multiply by on x x x 8 x 8 x 1 x 5 x 4 x 16

2 Using Inverse Operations with Powers and Roots Square Root Square Cube Root Cube Solve: x 4. Solve: x 4. Solve: x. Solve: x 8. x is being square rooted, so we square x is being squared, so we square root both sides. (using root) x is being cuberooted, so we cube x is being cubed, so we cube root both sides. x 4 x 4 x x 8 x 16 x or x x 8 x Note that undoing the square with a square root required both a positive and a negative in front of the root. That is because when we square a positive or a negative number we get a positive. We can t be sure if the x in x 4 should be a + or a - since both of these make the original equation true: 4 and 4. So, we include both + and - as an answer. Also note that we don t need the with the cube root since only a positive cubed would give us a positive. In other words, 8, but 8, so we just need the positive cube root. Example 1 Solve the following: a. x 9 b. c. x x 9 9 a. x 9 81 b. x 9 c. x 9 (this won t simplify, so we leave it as is)

3 Consider the following equation: x 1 There are two ways to solve this problem, and both of them require eliminating the parentheses. One method is to use the distributive property, and the other is to use inverse operations. The chart below shows a comparison of these techniques. Using Distributive Property Solve x 1. Distribute the through the parentheses x 6 1 Now use inverse operations by adding 6 to get the x isolated on the left side x x 18 Isolate the x by dividing both sides by x 18 Using Inverse Operations Only Solve x 1. Divide both sides by to isolate the parentheses x 1 x 4 Now we can remove the parentheses since it is alone on the left, then add on each side x 4 x 6 x 6 Now consider this similar problem: Solve x 1. This one cannot be solved by distributing the, we have to use inverse operations on this one. Example Solve x 1. x 1 x 4 x 4 x Divide both sides by Undo the square with a square root Add on both sides, simplify the root Simplify the + and the x 4 or x 0 Final answers!

4 To see why you can t distribute a square (or other powers), think about the following computations: The bottom line is obviously false, and so are all of the previous lines. The same is true for roots: It s important to remember that you CANNOT distribute a number through a power or a root, and you cannot distribute a power or a root to each term inside. This means we will only be using inverse operations to solve equations with powers or roots for now. Example Solve x 7 1. x 7 1 x 6 x 7 7 x x 7 Final answer! Subtract 7 on both sides to isolate the root term Divide both sides by to isolate the root Cube both sides to undo the cube - root Simplify Try this! Solve: x 5 Answer: x 7 4

5 Example 4 Solve x x 1 19 x 1 = 64 x 1 64 x 1 4 x x Divide by on both sides to isolate the square Take the cube root of both sides Simplify the root if possible Add 1 on both sides to isolate the x - term Divide by on both sides to isolate the x Final Answer! Example 5 and 6 are a couple of tougher examples where the roots don t simplify to nice whole numbers. Example 5 Solve x 100. x 100 x =50 Take the square root of both sides (+ and -) x 50 x 50 x 5 x 5 Divide by on both sides to isolate the square Add to both sides to isolate the x Simplify the root if possible Final Answer! 5

6 Example 6 Solve x x 5 48 x 5=16 x 5 16 x 5 56 x 56 5 x 51 Divide by on both sides to isolate the square - root Square both sides Simplify the square Subtract 5 on both sides to isolate the x Simplify Final Answer! The answers given above are exact answers since they are not rounded. You could also be asked for approximate answers as well, rounded to a certain number of decimals. The answers to Example 5 rounded to decimal places is shown below: Example 5: x 5 gives x or x 4.07 if you are asked for an answer rounded to two decimal places. KEY TAKEAWAYS Although we can t distribute like usual with a power or a root, we can solve some of these types of equations by undoing operations until we have isolated our variable. When solving a square by using a square-root, be sure to include the + and in front of the root. TOPIC EXERCISES Solve the following equations. 1. x 5 4. x 1 6. x 4. x x 4 x 5 6

7 x x x x x 8 1. x 8 x x x x x 18. x x x x x 81. 8x x x 4 x x 8 8. x 8 9. x x 1 Find the exact answer, then use a calculator to approximate to the nearest hundredth. 1. x 4. x x x x 0 6. x x 7 8. x 4 9. x x

8 ANSWERS 1.. x x 7, x 1 5. x x x 1, x x 5 1. x x x 8, x x x.. 4. x x 0, x x x 4 1. x 4,. Approx: x 1.74, x x 8 7, 4. Approx: x 10.65, x x 5, 6. Approx: x 7.47, x x 8. Approx: x 7.0, x x 5 4, Approx: x 0.66, x