The four properties that follow are not difficult to justify algebraically, but the details will not be presented here.
Property 1 (Means-Extremes Property, or Cross-Products Property): If a/b = c/d, then ad =bc. Conversely, if ad = bc ≠ 0, then 
and 
.
Example 1: Find a if a/12 = 3/4.
By Property 1:
Example 2: Is 3 : 4 = 7 : 8 a proportion?
No. If this were a proportion, Property 1 would produce
Property 2 (Means or Extremes Switching Property): If a/ b = c/ d and is a proportion, then both d/ b = c/ a and a/ c = b/ d are proportions.
Example 3: 8/10 = 4/5 is a proportion. Property 2 says that if you were to switch the 8 and 5 or switch the 4 and 10, then the new statement is still a proportion.
If 8/10 = 4/5, then 5/10 = 4/8, or if 8/10 = 4/5, then 8/4 = 10/5.
Example 4: If x/5 = y/4, find the ratio of x/ y.
Use the switching property of proportions and switch the means positions, the 5 and the y.
Property 3 (Upside-Down Property): If a/ b = c/ d, then b/ a = d/ c.
Example 5: If 9 a = 5 b ≠ 0, find the ratio 
.
First, apply the converse of the Cross Products Property and obtain the following:
Next, proceed in one of the following two ways:
Property 4 (Denominator Addition/Subtraction Property): If a/ b = c/ d, then ( a + b)/ b = ( c + d)/ d or ( a − b)/ b = ( c − d)/ d.
Example 6: If 5/8 = x/ y, then 13/8 = ?
Example 7: In Figure 1 , AB/ BC = 5/8. Find AC/ BC.
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| Figure 1 | Using the Segment Addition Postulate. | |
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Recall that AB + BC = AC (Segment Addition Postulate).
Example 8: A map is scaled so that 3 cm on the map is equal to 5 actual miles. If two cities on the map are 10 cm apart, what is the actual distance the cities are apart?
Let x = the actual distance.
Apply the Cross-Products Property.
The cities are 16⅔ miles apart.
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