ratios and proportions
DESCRIPTION
Algebra 1 tutorialTRANSCRIPT
Ratios and Proportions
Chapter 3 Section 6
Hmmmmm…… It was recently estimated by the Extraterrestrial
Alien Monitoring Agency that unregistered extraterrestrials outnumber registered ones by about seven-to-five. If there are a total of 2,280 registered extraterrestrials, about how many are unregistered?
Ratios
The expression seven-to-five is a ratio. It describes a size comparison between two things…..in this case, the number of registered aliens to unregistered ones.
Ways of expressing them….
• Ratios can be expressed with words, such as in the alien problem (seven-to-five)
• Ratios can be expressed with a colon (7:5)
• Ratios can be expressed as a fraction (7/5)
An Easy Example
The ratio of the middle alien’s toes to fingers could be written as :
1. three-to-four
2. 3:4
3. ¾
Cross Products (to determine proportions)
Two ratios can be proved equivalent (or Proportional) by using cross products:
(This is the ONLY time you will be able to CROSS MULTIPLY, so enjoy!)
ExamplesDetermine if the following
ratios are proportional (equivalent fractions):
11 & 7 5 3
Cross multiply and compare products:
11 x 3 = 335 x 7 = 35
How did the products compare?
If the products are equal, then the ratios are proportional.If the products are not equal, then the ratios are not proportional.
33 ≠ 35So 11 & 7 5 3 are NOT proportional!
Your turn:
Use cross products to determine if the following ratios are proportional:
4 & 12 11 33
Answer (work shown!)
Cross products are:
4 x 33 = 132
11 x 12 = 132
132 = 132, so they ARE proportional!
Unknown values in a Proportion:
Sometimes you will be given a proportion (equal fractions) with an unknown value to solve for, such as the following:
3 = x 13
52
Use cross products to solve for x:
13x = 3(52)
13x = 156 Divide both
sides by 13:
X = 12
Your turn:
Use cross products to solve for h:
20 = h
28 21
Pretty easy, right?!?Answer:
28h = 20(21)
28h = 420
28 28
h = 15
What happens if there are DECIMALS in the
problem?!?!
USE YOUR CALCULATOR
USE CROSS PRODUCTS
GET THE ANSWER!
Now, solve the alien problem using ratios /
proportions:It was recently estimated by the Extraterrestrial
Alien Monitoring Agency that unregistered extraterrestrials outnumber registered ones by about seven-to-five. If there are a total of 2,280 registered extraterrestrials, about how many are unregistered?
Set-Up:• The given ratio is seven-to-five
(unregistered to registered)
• The other given amount is 2,280 registered
• The unknown amount is for unregistered
7 = _u_
5 2280
Cross Multiply & Divide:
7 = _u_
5 2280
5u = 7(2280)
5u = 15960
5 5
u = 3192
Assignment:
Worksheet 3-62 sides!!
The End!