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# Adding Fractions with Different Denominators - PowerPoint PPT Presentation

Adding Fractions with Different Denominators. (mostly the how, a little about the why or when). 3/8 + 4/9 Step one: “what is this problem asking me to do?” Add fractions, which means what? You need a common denominator. (Multiplication and division *don’t*) .

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## PowerPoint Slideshow about ' Adding Fractions with Different Denominators' - rafael

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### Adding Fractions with Different Denominators

(mostly the how, a little about the why or when)

• 3/8 + 4/9

• Step one: “what is this problem asking me to do?”

• Add fractions, which means what?

• You need a common denominator. (Multiplication and division *don’t*)

• Welp, if I said I wanted to add 8 inches and 3 feet…

• Would that be 11 miles?

• I don’t think so.

• It wouldn’t be 11 inches… it wouldn’t be 11 feet…

• It would be 3 feet and 8 inches…

• One foot is exactly the same as 12 inches.

• 3 feet would have 12 + 12 + 12 inches, or 3 x 12 inches.

• 36 inches plus the other eight inches would mean we had 44 inches total.

• We were adding things of the same size.

3/8 -------

4/9 -----

Put ‘em together…Huh???? It isn’t eigths or ninths…

4/9

3/8

• Think of the denominator as shoes.

• If the fractions aren’t wearing the same kinds of shoes, they can’t dance together.

• Sorry, those are the rules  (and I did explain why, remember?)

• OR… since you’ve been working with “like terms”… the denominator is like an “x” or a “y.” 3/8 + 4/9 is like adding 3x and 4y (but x would be 1/8 and 7 would be 1/9)… you can’t just put ‘em together.

Rewrite the Problem Vertically denominator

3

8

+ 4

9

Find the Common denominatorDenominator.Write it in.

• (You’re not *changing* the fraction, just its name. 2 quarters is worth the same amount as 5 dimes or 10 nickels; they just look different.)

3 ___

8 72

+ 4 ___

9 72

If you’re not sure what the *least* common denominator is, you can always *multiply the two denominators.*

3 x9 ___

8 x9 72

+ 4 x8 ___

9 x8 72

To keep the fractions equivalent, treat the numerator the same as the denominator for each fraction.

3 x9 27

8 x9 72

+ 4 x8 32

9 x8 72

Add the numerators, and keep that common denominator. same as the denominator for each fraction.

3 x9 27

8 x9 72

+ 4 x8 32

9 x8 72

59

72

(Reduce it if you can. You can’t )

• Find and write Common Denominator same as the denominator for each fraction.

• Find the multiplication and write it down

• Multiply across

• Reduce

• … when you’re an expert, you can skip copying the “x 8 x 8 x 9 x 9” part.

x9 same as the denominator for each fraction.

x9

x9

72

x9

+

x8

+

x8

x8

72

x8

Write in the multiplication, TOP AND BOTTOM of fraction

(I do it bottom-up)

Copy Vertically

Add the top numbers. Bottom one is the “kind of shoe” – it stays the same!

x9

72

x9

+

x8

72

Reduce if you can… but you can’t this time 

x8

Write in Common Denominator (multiplying them always works)

Multiply to get

New Numerators (finish the circle)