Core Connections Geometry, 2013
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Core Connections Geometry, 2013 View details
2. Section 12.2
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Exercise 71 Page 756

Practice makes perfect
a To calculate the measure of an interior angle of a regular n-gon, we can use the formula 180^(∘)(n-2)n, where n is the number of sides in the polygon. A pentagon is a polygon with 5 sides.
180^(∘)(n-2)/n
180^(∘)( 5-2)/5
Evaluate
180^(∘)(3)/5
540^(∘)/5
108^(∘)
The interior angle of a regular pentagon is 108^(∘).

Next, we will draw an equilateral triangle with a central angle of a.

The sum of the central angles in any regular polygon equals 360^(∘). Since an equilateral triangle has 3 congruent central angles, we can determine the measure of a by dividing 360^(∘) by 3. 360^(∘)/3= 120^(∘) As we can see, a=120^(∘) and b=108^(∘). Therefore a is greater than b.

b Notice that a and b are corresponding angles. If the two lines cut by the transversal were parallel, we could claim that a=b by the Corresponding Angles Theorem.

However, we do not have this information. Therefore, we have no way of saying which angle is greater or if they are the same.

c With the given information, we can calculate ∠ a with the sine ratio and ∠ b with the cosine ratio.
Let's solve these equations for a and b.
sin a=8/12
Solve for a
sin a=2/3

sin^(-1)(LHS) = sin^(-1)(RHS)

a=sin^(-1)2/3
a=41.8103148
a≈ 41.81^(∘)
Let's also solve for b in the second equation.
cos b=8/12
Solve for b
cos b=2/3

cos^(-1)(LHS) = cos^(-1)(RHS)

b=cos^(-1)2/3
b=48.18968...
b≈ 48.19^(∘)
From our calculations we see that b is greater than a.
d The equation tells us that to obtain a we have to add 3 to b. This must mean that a is greater than b.

a = b + 3 a is 3 greater than b

e Like in Part A, we can use trigonometric ratios to write equations containing a and b. For the triangle on the left we need to use the tangent ratio. In the triangle on the right, we need to use the cosine ratio.
Let's solve these equations for a and b.
tan 23^(∘)=a/7
Solve for a
7tan 23^(∘)=a
a=7tan 23^(∘)
a=2.97132...
a≈ 2.97
Let's also solve for b in the second equation.
cos 49^(∘)=2/b
Solve for b
bcos 49^(∘)=2
b=2/cos 49^(∘)
b=3.04850...
b≈ 3.05
From our calculations we see that b is greater than a.