McGraw Hill Integrated II, 2012
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McGraw Hill Integrated II, 2012 View details
6. The Law of Sines and Law of Cosines
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Exercise 42 Page 676

Begin by using the Law of Cosines.

m ∠ X ≈ 74
m ∠ Y ≈ 23
m ∠ Z ≈ 83

Practice makes perfect

Let's begin by drawing △ XYZ and labeling the lengths of the sides. We will also color code the opposite angles and sides. It will help us use the Law of Sines and Law of Cosines later.

First, we can tell that it is not a right triangle, as the sides do not satisfy the Pythagorean Theorem. 184^2+ 75^2 ≠ 190^2 Let's find the measures of ∠ X, ∠ Y, and ∠ Z one at a time.

Finding m ∠ X

The measures of all three sides of the triangle are given. Therefore, we can use the Law of Cosines to find m ∠ X. x^2=y^2+z^2 -2 y z cos X Let's substitute x= 184, y= 75, and z= 190 to isolate cos X.
x^2=y^2+z^2 -2 y z cos X
184^2= 75^2+ 190^2 -2 ( 75)( 190) cos X
Solve for cos X
33856=5625+36100 - 2(75)(190)cos X
33856=5625+36100 - 28500 cos X
33856 = 41725- 28500 cos X
28500 cos X +33856 = 41725
28500 cos X =7869
cos X =7869/28500
Now, we can use the inverse cosine ratio and a calculator to find m ∠ X.
m ∠ X = cos ^(-1) 7869/28500
m ∠ X = 73.972108...
m ∠ X ≈ 74

Finding m ∠ Y

Now that we know the measure of ∠ X, we can find m ∠ Y using the Law of Sines. sin X/x =sin Y/y Let's substitute x= 184, m ∠ X ≈ 74, and y= 75, to isolate sin Y.
sin X/x =sin Y/y
sin 74/184 = sin Y/75
Solve for sin Y
75 sin 74/184 = sin Y
sin Y =75 sin 74/184
Now we can use the inverse sine ratio to find m ∠ Y.
m ∠ Y = sin ^(-1) 75 sin 74/184
m ∠ Y = 23.067707...
m ∠ Y ≈ 23

Finding m ∠ Z

Finally, to find m ∠ Z we can use the Triangle Angle Sum Theorem. This tells us that the measures of the angles in a triangle add up to 180. 74+ 23 + m ∠ Z = 180 ⇔ m ∠ Z ≈ 83

Completing the Triangle

With all of the angle measures, we can complete our diagram.