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Solving Equations 8 th Grade

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Table of Contents Inverse Operations One Step Equations Two Step Equations Multi-Step Equations More Equations Transforming Formulas Click on a topic to go to that section.

Inverse Operations Return to Table of Contents

What is an equation? An equation is a mathematical statement, in symbols, that two things are exactly the same (or equivalent). Equations are written with an equal sign, as in = 5 9 – 2 = 7

Equations can also be used to state the equality of two expressions containing one or more variables. In real numbers we can say, for example, that for any given value of x it is true that 4x + 1 = If x = 3, then 4(3) + 1 = = = 13

When defining your variables, remember... Letters from the beginning of the alphabet like a, b, c... often denote constants in the context of the discussion at hand. While letters from end of the alphabet, like x, y, z..., are usually reserved for the variables, a convention initiated by Descartes. Try It! Write an equation with a variable and have a classmate identify the variable and its value.

An equation can be compared to a balanced scale. Both sides need to contain the same quantity in order for it to be "balanced".

For example, = 50 represents an equation because both sides simplify to = = 50 Any of the numerical values in the equation can be represented by a variable. Examples: 20 + c = 50 x + 30 = = y

Why are we Solving Equations? First we evaluated expressions where we were given the value of the variable and had to find what the expression simplified to. Now, we are told what it simplifies to and we need to find the value of the variable. When solving equations, the goal is to isolate the variable on one side of the equation in order to determine its value (the value that makes the equation true).

In order to solve an equation containing a variable, you need to use inverse (opposite/undoing) operations on both sides of the equation. Let's review the inverses of each operation: Addition Subtraction Multiplication Division

There are four properties of equality that we will use to solve equations. They are as follows: Addition Property If a=b, then a+c=b+c for all real numbers a, b, and c. The same number can be added to each side of the equation without changing the solution of the equation. Subtraction Property If a=b, then a-c=b-c for all real numbers a, b, and c. The same number can be subtracted from each side of the equation without changing the solution of the equation. Multiplication Property If a=b, and c=0, then ac=bc for all real numbers ab, b, and c. Each side of an equation can be multiplied by the same nonzero number without changing the solution of the equation. Division Property If a=b, and c=0, then a/c=b/c for all real numbers ab, b, and c. Each side of an equation can be divided by the same nonzero number without changing the solution of the equation.

To solve for "x" in the following equation... x + 7 = 32 Determine what operation is being shown (in this case, it is addition). Do the inverse to both sides. x + 7 = x = 25 In the original equation, replace x with 25 and see if it makes the equation true. x + 7 = = = 32

For each equation, write the inverse operation needed to solve for the variable. a.) y + 7 = 14 subtract 7 b.) a - 21 = 10 add 21 c.) 5s = 25 divide by 5 d.) x = 5 multiply by move

Think about this... To solve c - 3 = 12 Which method is better? Why? Kendra Added 3 to each side of the equation c - 3 = c = 15 Ted Subtracted 12 from each side, then added 15. c - 3 = c - 15 = c = 15

Think about this... In the expression To which does the "-" belong? Does it belong to the x? The 5? Both? The answer is that there is one negative so it is used once with either the variable or the 5. Generally, we assign it to the 5 to avoid creating a negative variable. So: Touch to reveal answer

1What is the inverse operation needed to solve this equation? 7x = 49 AAddition BSubtraction CMultiplication DDivision

2What is the inverse operation needed to solve this equation? x - 3 = -12 AAddition BSubtraction CMultiplication DDivision

One Step Equations Return to Table of Contents

To solve equations, you must work backwards through the order of operations to find the value of the variable. Remember to use inverse operations in order to isolate the variable on one side of the equation. Whatever you do to one side of an equation, you MUST do to the other side!

Examples: y + 9 = The inverse of adding 9 is subtracting 9 y = 7 6m = The inverse of multiplying by 6 is dividing by 6 m = 12 Remember - whatever you do to one side of an equation, you MUST do to the other!!!

x - 8 = x = 6 x + 2 = x = = x = x 7 = x = x 15 = x = x x + 5 = x = -2 One Step Equations Solve each equation then click the box to see work & solution. click to show inverse operation click to show inverse operation click to show inverse operation click to show inverse operation click to show inverse operation click to show inverse operation

One Step Equations 3x = x = 5 -4x = x = = 5x = x click to show inverse operation click to show inverse operation click to show inverse operation x 2 x = 20 = 10 (2) x -6 x = -216 = 36 click to show inverse operation (-6) click to show inverse operation

3Solve. x - 6 = -11

4Solve. j + 15 = -17

5Solve = -5x

6 Solve. = 12 x 9

7 Solve. 51 = 17y

8Solve. w - 17 = 37

9 Solve. -3 = x 7

10Solve t = 11

11Solve. 108 = 12r

Two-Step Equations Return to Table of Contents

Sometimes it takes more than one step to solve an equation. Remember that to solve equations, you must work backwards through the order of operations to find the value of the variable. This means that you undo in the opposite order (PEMDAS): 1st: Addition & Subtraction 2nd: Multiplication & Division 3rd: Exponents 4th: Parentheses Whatever you do to one side of an equation, you MUST do to the other side!

Examples: 3x + 4 = Undo addition first 3x = Undo multiplication second x = 2 -4y - 11 = Undo subtraction first -4y = Undo multiplication second y = 3 Remember - whatever you do to one side of an equation, you MUST do to the other!!! Touch to reveal answer

6-7x = x = x = -11 3x + 10 = x = x = 12 -4x - 3 = x = x = -7 -2x + 3 = x = x = x = x = x = x = x = x = 8 Two Step Equations Solve each equation then click the box to see work & solution.

Walter is a waiter at the Towne Diner. He earns a daily wage of $50, plus tips that are equal to 15% of the total cost of the dinners he serves. What was the total cost of the dinners he served if he earned $170 on Tuesday? From the New York State Education Department. Office of Assessment Policy, Development and Administration. Internet. Available from accessed 17, June, 2011.

12Solve the equation. 5x - 6 = -56

13Solve the equation. 16 = 3m - 8

14Solve the equation.

15Solve the equation. 5r - 2 = -12

16Solve the equation. 12 = -2n - 4

17Solve the equation.

18Solve the equation.

19What is the value of n in the equation 0.6(n + 10) = 3.6? A B C From the New York State Education Department. Office of Assessment Policy, Development and Administration. Internet. Available from accessed 17, June, D4

20In the equation n is equal to? A B C D From the New York State Education Department. Office of Assessment Policy, Development and Administration. Internet. Available from accessed 17, June,

21 Which value of x is the solution of the equation ? A B C D From the New York State Education Department. Office of Assessment Policy, Development and Administration. Internet. Available from accessed 17, June, 2011

22Two angles are complementary. One angle has a measure that is five times the measure of the other angle. What is the measure, in degrees, of the larger angle? From the New York State Education Department. Office of Assessment Policy, Development and Administration. Internet. Available from accessed 17, June, 2011.

Multi-Step Equations Return to Table of Contents

Steps for Solving Multiple Step Equations As equations become more complex, you should: 1. Simplify each side of the equation. (Combining like terms and the distributive property) 2. Use inverse operations to solve the equation. Remember, whatever you do to one side of an equation, you MUST do to the other side!

Examples: -15 = -2x x -15 = 2x - 9 Combine Like Terms Undo Subtraction first -6 = 2x 2 2 Undo Multiplication second -3 = x 7x - 3x - 8 = 24 4x - 8 = 24Combine Like Terms Undo Subtraction first 4x = Undo Multiplication second x = 8 Touch to reveal answer

Now try an example. Each term is infinitely cloned so you can pull them down as you solve. -7x x = -6 ans wer -7x x -6

Now try another example. Each term is infinitely cloned so you can pull them down as you solve. 6x x = 44 ans wer 6x x 44

Always check to see that both sides of the equation are simplified before you begin solving the equation. Sometimes, you need to use the distributive property in order to simplify part of the equation.

For all real numbers a, b, c a(b + c) = ab + ac a(b - c) = ab - ac Distributive Property

Examples 5(20 + 6) = 5(20) + 5(6) 9(30 - 2) = 9(30) - 9(2) 3(5 + 2x) = 3(5) + 3(2x) -2(4x - 7) = -2(4x) - (-2)(7)

Examples: 5(1 + 6x) = x = 185Distribute the 5 on the left side Undo addition first 30x = Undo multiplication second x = 6 2x + 6(x - 3) = 14 2x + 6x - 18 = 14Distribute the 6 through (x - 3) 8x - 18 = 14 Combine Like Terms Undo subtraction 8x = Undo multiplication x = 4 Move to reveal answer

5 (-2 + 7x) = 95 Now show the distributing and solve...(each number/ symbol is infinitely cloned, so click on it and drag another one down)

6 ( -2x + 9 ) = 102 Now show the distributing and solve...(each number/ symbol is infinitely cloned, so click on it and drag another one down)

23Solve t + 4t = -63

24Solve. 19 = x

25Solve. 8x x - 11 = -27

26Solve. -4 = -27y (-15y) + 13

27Solve y y = 4

28Solve. 6(-8 + 3b) = 78

29Solve. 18 = -6(1 - 1k)

30Solve. 2w + 8(w + 3) = 34

31Solve. 4 = 4x - 2(x + 6)

32Solve. 3r - r + 2(r + 4) = 24

33What is the value of p in the equation 2(3p - 4) = 10? A1 B2 1/3 C3 D1/3 From the New York State Education Department. Office of Assessment Policy, Development and Administration. Internet. Available from accessed 17, June, 2011.

Return to Table of Contents More Equations

Remember Simplify each side of the equation. 2. Solve the equation. (Undo addition and subtraction first, multiplication and division second) Remember, whatever you do to one side of an equation, you MUST do to the other side!

5 3 Examples: x = 6 x = 6 Multiply both sides by the reciprocal x = x = 10 2x - 3 = + x -x - x Subtract x from both sides x - 3 = Undo Subtraction x = Click to reveal steps

There is more than one way to solve an equation with distribution. Multiply by the reciprocal Multiply by the LCM (-3 + 3x) = (-3 + 3x) = (-3 + 3x) = (-3 + 3x) = x = x = x = 9 (-3 + 3x) = (-3 + 3x) = x = x = x = 9

34Solve

35Solve

36Solve

37Solve

7(2x +9) = -3(21) 38Solve

39Solve

Transforming Formulas Return to Table of Contents

Formulas show relationships between two or more variables. You can transform a formula to describe one quantity in terms of the others by following the same steps as solving an equation.

Example: Transform the formula d = r  t to find a formula for time in terms of distance and rate. What does "time in terms of distance and rate" mean? d = r  t r = t d r Divide both sides by r Slide to reveal steps

Examples V = l whSolve for w V = w l h P = 2l + 2w Solve for l -2w P - 2w = 2l 2 2 P - 2w = l 2 Slide to reveal steps Slide to reveal steps

Example: To convert Fahrenheit temperature to Celsius, you use the formula: C = (F - 32) Transform this formula to find Fahrenheit temperature in terms of Celsius temperature. (see next page) 5 9

C = (F - 32) C = F - + C + = F C + 32 = F () 9 5 Solve the formula for F Slide to reveal steps

Transform the formula for area of a circle to find radius when given Area. A = r 2 = r 2 A = r A Slide to reveal answer

Solve the equation for the given variable. m p n q m p n q mq p n = for p =(q) = (q) 2(t + r) = 5 for t 2(t + r) = t + r = - r - r t =  r Move to reveal steps Move to reveal steps

40The formula I = prt gives the amount of simple interest, I, earned by the principal, p, at an annual interest rate, r, over t years. Solve this formula for p. Ap = B C D l rt Irt Ir t It r

41A satellite's speed as it orbits the Earth is found using the formula. In this formula, m stands for the mass of the Earth. Transform this formula to find the mass of the Earth. Am = B C D rv 2 G v 2 = Gm r v 2 - r G rv 2 - G v2Gv2G - r

42Solve for t in terms of s 4(t - s) = 7 At = + s Bt = 28 + s Ct = - s Dt = s 4

43 Solve for w A = lw Aw = Al B w = C A l A

44Solve for h A B C D

45Which equation is equivalent to 3x + 4y = 15? Ay = 15 − 3x By = 3x − 15 Cy = 15 – 3x 4 Dy = 3x – 15 4 From the New York State Education Department. Office of Assessment Policy, Development and Administration. Internet. Available from accessed 17, June, 2011.

46 If, b ≠ 0, then x is equal to A B C D From the New York State Education Department. Office of Assessment Policy, Development and Administration. Internet. Available from accessed 17, June, 2011