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Solving an Equation

Solving an Equation. Finding ALL possible values that make an equation true . You are done solving an equation when you can look at an equation and “ know ” what values make it true.

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Solving an Equation

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  1. Solving an Equation • Finding ALL possible values that make an equation true. • You are done solving an equation when you can look at an equation and “know” what values make it true. • If you can’t look at an equation and “know” what values make it true, try rearranging the equation to make it easier to look at.

  2. Examples

  3. Examples x=2 x=1/3 2x=10; x=5

  4. Solving an Equation • Finding ALL possible values that make an equation true. • You are done solving an equation when you can look at an equation and “know” what values make it true. • If you can’t look at an equation and “know” what values make it true, try rearranging the equation to make it easier to look at.

  5. Ways to rearrange equations • Properties of arithmetic • Associative, Commutative, and Distributive • Use these to get rid of parentheses and combine like terms • These change the shape of one side of the equation, but not its value.

  6. Doing the same thing to both sides • Adding, Subtracting, Multiplying, Dividing, a number from both sides of the equation. • Changes the value of both sides, but not the equality.

  7. WARNING • Each side is a number. When multiplying (or dividing) multiply (or divide) the whole number.

  8. Rearranging an equation to solve

  9. Solve for K: 2(K-7) = 4 - 3(K+2) • K=12/5 • K=5/24 • K=24/5 • None of the above

  10. Solve for K: 2(K-7) = 4 - 3(K+2) A

  11. Solve an inequality • Method 1: Convert to an equality, solve, then test. • Slower • Works for complex problems later in the course • Method 2: Solve the inequality like an equality • Faster • Easier to make mistakes • Does not work for complicated problems like quadratic inequalities.

  12. Method 1 • Start with inequality: 8<4-2x • Convert to an equality: 8=4-2x • Solve: -2=x • Draw a number line <------------|------------------> x • x<-2 -2 x>-2 • Your possible answers are x<-2 and x>-2. Pick a number on each side to test which is correct. • Test (x<-2) with x=-3: 8<4-2(-3) 8<10 TRUE • Test (x>-2) with x=0: 8<4-2(0) 8<4 FALSE • ANSWER: x<-2

  13. Method 2 • Start with inequality: 8<4-2x • Solve by doing the same thing to both sides 8+2x<4 • 2x<4-8 • 2x<-4 • x<-2

  14. PROBLEM • Using method 2, if I do this problem two different ways, I get two different answers. DON’T MATCH!

  15. Which is Correct? • I know x<-2 is correct, because when I did method 1, I plugged in to check my work. CORRECT INCORRECT

  16. How did it happen? • When I turned -2x into +2x, I moved it to the other side of the <. When I divided by -2, I turned negative into positive, but didn’t move it to the other side. Moved x Never moved x.

  17. How to fix it • When I divide by a negative, I can have the same effect as “moving to the other side” by “flipping the sign” Moved x Answers Match Flipped the sign

  18. NEW RULE • When multiplying or dividing an inequality by a negative number, flip the sign of the inequality • Or, just never multiply by a negative.

  19. Solve the following inequality: -3(x+4) > 53 • x > 65/3 • x > -65/3 • x < 65/3 • x < -65/3 • None of the above

  20. Solve the following inequality: -3(x+4) > 53 D

  21. How to set up word problems

  22. How to set up a word problem(with equation given) • A jewelry maker has total revenue for necklaces given by R(x)=90.75x, and incurs a total cost of C(x)=24.50x+4770, where x is the number of necklaces made and sold. How many necklaces must be produced and sold in order to break even?

  23. Step 1: Define the variables • A jewelry maker has total revenue for necklaces given by R(x)=90.75x, and incurs a total cost of C(x)=24.50x+4770, where x is the number of necklacesmade and sold. How many necklaces must be produced and sold in order to break even? • x=number of necklaces • R(x)=number of $ earned for x necklaces • C(x)=number of $ spend to make x necklaces

  24. Step 2: Interpret the question • A jewelry maker has total revenue for necklaces given by R(x)=90.75x, and incurs a total cost of C(x)=24.50x+4770, where x is the number of necklaces made and sold. How many necklaces must be produced and sold in order to break even? • x=number of necklaces • R(x)=number of $ earned for x necklaces • C(x)=number of $ spend to make x necklaces • x=? • R(x)=C(x)

  25. Step 3: Substitute • A jewelry maker has total revenue for necklaces given by R(x)=90.75x, and incurs a total cost of C(x)=24.50x+4770, where x is the number of necklaces made and sold. How many necklaces must be produced and sold in order to break even? • x=number of necklaces • R(x)=number of $ earned for x necklaces • C(x)=number of $ spend to make x necklaces • x=? • R(x)=C(x) • 90.75x=24.50x+4770

  26. DONE! • A jewelry maker has total revenue for necklaces given by R(x)=90.75x, and incurs a total cost of C(x)=24.50x+4770, where x is the number of necklaces made and sold. How many necklaces must be produced and sold in order to break even? • x=number of necklaces • R(x)=number of $ earned for x necklaces • C(x)=number of $ spend to make x necklaces • x=? • R(x)=C(x) • 90.75x=24.50x+4770 solve for x.

  27. How to set up a word problem(without an equation) • Suppose that in a certain business, there were 30 employees in the year 1920, and 38 employees in the year 1940. Assuming linear growth, In what years will the business have more than 40 employees?

  28. Step 1: Identify your variables • Suppose that in a certain business, there were 30 employees in the year 1920, and 38 employees in the year 1940. Assuming linear growth, In what years will the business have more than 40 employees? • P=number of employees • t=number of years

  29. Step 2: Identify your Points • Suppose that in a certain business, there were 30 employees in the year 1920, and 38 employees in the year 1940. Assuming linear growth, In what years will the business have more than 40 employees? • P=number of employees • t=number of years [Put independent variable first] • (t,P): (1920,30), (1940,38)

  30. Step 3: Interpret your question • Suppose that in a certain business, there were 30 employees in the year 1920, and 38 employees in the year 1940. Assuming linear growth, In what years will the business have more than 40 employees? • P=number of employees • t=number of years • (t,P): (1920,30), (1940,38) • t=? • P>40?

  31. Step 4: Choose your generic formula • Suppose that in a certain business, there were 30 employees in the year 1920, and 38 employees in the year 1940. Assuming linear growth, In what years will the business have more than 40 employees? • P=number of employees • t=number of years • (t,P): (1920,30), (1940,38) • t=? • P>40? • (y-b)=m(x-a)

  32. Step 5: Match variables and plug in numbers • Suppose that in a certain business, there were 30 employees in the year 1920, and 38 employees in the year 1940. Assuming linear growth, In what years will the business have more than 40 employees? • P=number of employees • t=number of years [Make independent variable x] • (t,P): (1920,30), (1940,38) • t=? • P>40? • (y-b)=m(x-a) • (P-30)=[(38-30)/(1940-1920)](t-1920)

  33. Step 6: Rearrange and substitute • Suppose that in a certain business, there were 30 employees in the year 1920, and 38 employees in the year 1940. Assuming linear growth, In what years will the business have more than 40 employees? • P=number of employees • t=number of years [Make independent variable x] • (t,P): (1920,30), (1940,38) • t=? • P>40? • y=m(x-a)+b • (P-30)=[8/20](t-1920) • P=[8/20](t-1920)+30>40

  34. DONE! • Suppose that in a certain business, there were 30 employees in the year 1920, and 38 employees in the year 1940. Assuming linear growth, In what years will the business have more than 40 employees? • P=number of employees • t=number of years [Make independent variable x] • (t,P): (1920,30), (1940,38) • t=? • P>40? • y=m(x-a)+b • (P-30)=[(38-30)/(1940-1920)](t-1920) • P=[8/20](t-1920)+30>40, solve for t.

  35. A plumbing store’s monthly profit from the sale of PVC pipe can be described by P(x)=5.25x-1150 dollars, where x is the number of feet of PVC pipe sold. Set up an inequality that would help determine what level of monthly sales is necessary to incur positive profit. • 5.25x >1150 • 5.25x+1150>0 • 5.25x+1150=0 • Both A & B • All of the above

  36. A plumbing store’s monthly profit from the sale of PVC pipe can be described by P(x)=5.25x-1150 dollars, where x is the number of feet of PVC pipe sold. Set up an inequality that would help determine what level of monthly sales is necessary to incur positive profit. Define Variables P(x)=number of dollars of profit x=number of feet of pipe sold Interpret Question x=? P(x)>0 Substitute 5.25x-1150>0 A) 5.25x>1150

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