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Danny
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Writing and Solving Proportions
• I can write and solve proportions using ratios
and algebra.
• Textbook Pages 412-416
Vocabulary
• Proportion: is an equation that shows that two
ratios are equivalent.
Ratio: 2 of 3 rows
Ratio: 8 of 12 dots
Proportion : 2/3 = 8/12
Cross Products
• Cross Product: when you multiply the numerator
of each ratio by the denominator of the other
ratio.
• The cross products of proportions are equal to
each other.
To see if two ratios form a proportion, check to see
if the cross products are equal:
8 20
10 25 so multiply 25 time 8 and 10 time 20
What you get is 200=200
Try some
Are these proportions:
10/16 = 5/8
Yes
3/8 = 15/40
Yes
3/5=12/18
No
Solving Using Mental Math:
• When one of the numbers in a proportion is
unknown, you can find the number by solving the
proportion.
• Just ask yourself what did they do to one to get
the other.
• 4 = 20
12
x
Multiply by 5
X= 60
Try some
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•
•
•
•
•
1/5 = z/20
4
8/3 = k/18
48
9/c=3/12
36
Review
• Can someone remind me how we solve
multiplication equations:
• We divide both sides by whatever number is
with the variable.
• 3x = 18
• Divide both sides by 3
• X=6
Solving using equations:
• You can solve proportions by using cross
product and making an equation.
• You cross multiply then solve for the variable.
• 10
50
60
x
Cross multiply.
10x = 3000 Solve by dividing both sides by 10
X=300
You try some!
• 91/x = 7/5
• 7x= 455
X=65
24/36 = 10/x
24x=360
X= 15
9/6 = h/14
6h=126
H=21
Practice
• Workbook Pages 105-106 odds