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

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.

Examples

xx

x

x

x

28)1(4

732

13

2

Examples

x=2

x=1/3

2x=10; x=5

xx

x

x

x

28)1(4

732

13

2

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.

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.

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.

WARNING

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

Rearranging an equation to solve

2

42

244

2844

28)1(4

22

44

x

x

xx

xx

xx

xx

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

A) K=12/5B) K=5/24C) K=24/5D) None of the above

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

A

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.

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

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

PROBLEM

• Using method 2, if I do this problem two different ways, I get two different answers.

DON’T MATCH!

Which is Correct?

• I know x<-2 is correct, because when I did method 1, I plugged in to check my work.

CORRECT INCORRECT

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.

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

Flipped the sign

Answers Match

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.

Solve the following inequality:

-3(x+4) > 53

A) x > 65/3

B) x > -65/3

C) x < 65/3

D) x < -65/3

E) None of the above

Solve the following inequality:

-3(x+4) > 53

D

How to set up word problems

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?

Step 1: Define the variables


• x=number of necklaces• R(x)=number of $ earned for x necklaces• C(x)=number of $ spend to make x necklaces

Step 2: Interpret the question


• 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)

Step 3: Substitute


• 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

DONE!


• 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.

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?

Step 1: Identify your variables


• P=number of employees• t=number of years

Step 2: Identify your Points


• P=number of employees• t=number of years [Put independent variable first]• (t,P): (1920,30), (1940,38)

Step 3: Interpret your question


• P=number of employees• t=number of years• (t,P): (1920,30), (1940,38)• t=?• P>40?

Step 4: Choose your generic formula


• P=number of employees• t=number of years• (t,P): (1920,30), (1940,38)• t=?• P>40?• (y-b)=m(x-a)

Step 5: Match variables and plug in numbers


• 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)

Step 6: Rearrange and substitute


• 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

DONE!


• 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.

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.

A) 5.25x >1150

B) 5.25x+1150>0

C) 5.25x+1150=0

D) Both A & B

E) All of the above

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 VariablesP(x)=number of dollars of profitx=number of feet of pipe sold

Interpret Questionx=?P(x)>0

Substitute5.25x-1150>0

A) 5.25x>1150

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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