Do ratios form a proportion?

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Do ratios form a proportion?
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What is a ratio?

• A ratio compares values.
• Above you see 3 blue squares and 1 red square.
• This can be written 3 different ways to show it
  is a ratio
• 31
• 3 to 1

Another example
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Scale up or scale down . . .

• A ratio can always be scaled up or scaled down.
• For example
• 31 is the same as 62
• It may be easier to see if you think of it like
  equivalent fractions and set them equal

Here the numerator (top) and denominator
(bottom) are both multiplied by 2 so the ratio is
still the same.
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Proportion

• A proportion says that two ratios are equal.
• Example
• Another example

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How do you know if ratios form a proportion?

• Remember a proportion is a statement that two
  ratios are equal.
• When you are given a problem the way to check to
  see if they are proportional is to cross
  multiply
• 20 5 100
• 25 4 100

• We need to check to see if these are
  proportional.
• First cross multiply
• Then we see if we get the same number.
• If we have the same number they are proportional.

100 100 So these are proportional!
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Lets look at a few more examples . . .

• Are the ratios below proportional?
• Cross multiply first
• 8 40 320
• 10 32 320
• You end up with the same number.

• Are the ratios below proportional?
• Cross multiply first
• 10 5 50
• 20 4 80
• You do not have the same number.

320320 So they are proportional!
50 ? 80 So they are NOT proportional!