The trigonometry of non-right triangles
So far, we've only dealt with right triangles, but trigonometry can be easily applied toĀ non-right trianglesĀ because any non-right triangle can be divided by anĀ altitude*Ā into two right triangles.
Roll over the triangle to see what that means ā
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Remember that an altitude is a line segment that has one endpoint at a vertex of a triangle intersects the opposite side at a right angle. SeeĀ triangles.
Customary labeling of non-right triangles
This labeling scheme is comĆmonly used for non-right triangles.Ā Capital lettersĀ areĀ anglesand the correspondingĀ lower-case lettersĀ go with theĀ sideĀ opposite the angle: sideĀ aĀ (with length ofĀ aĀ units) is across from angleĀ AĀ (with a measure ofĀ AĀ degrees or radians), and so on.
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Derivation of the law of sines
Consider the triangle below. if we find the sines of angleĀ AĀ and angleĀ CĀ using their corresponding right triangles, we notice that they both contain the altitude, x.
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The sine equations are
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We can rearrange those by solving each forĀ x(multiply byĀ cĀ on both sides of the left equation, and byĀ aĀ on both sides of the right):
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Now the transitive property says that if bothĀ cĀ·sin(A)Ā andĀ aĀ·sin(C)Ā are equal toĀ x, then they must be equal to each other:
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We usually divide both sides byĀ acĀ to get the easy-to-remember expression of the law of sines:
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We could do the same derivation with the other two altitudes, drawn from anglesĀ AĀ andĀ CĀ to come up with similar relations for the other angle pairs. We call these together the law of sines. It's in the green box below.
The law of sines can be used to find the measure of an angle or a side of a non-right triangle if we know:
two sides and an angle not between them ortwo angles and a side not between them.
Law of Sines
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Examples: Law of sines
Use the law of sines to find the missing measurements of the triangles in these examples. In the first, two angles and a side are known. In the second two sides and an angle. Notice that we need to know at least one angle-opposite side pair for the Law of Sines to work.
Example 1
Find all of the missing measurements of this triangle:
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The missing angle is easy, it's just
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Now set up one of the law of sines proportions and solve for the missing piece, in this case the length of the lower side:
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Then do the same for the other missing side. It's best to use the original known angle and side so that round-off errors or mistakes don't add up.
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Example 2
Find all of the missing measurements of this triangle:
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First, set up one law of sines proportion. This time we'll be solving for a missing angle, so we'll have to calculate anĀ inverse sine:
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Now it's easy to calculate the third angle:
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Then apply the law of sines again for the missing side. We have two choices, we can solve
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Either gives the same answer,
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Derivation of the law of cosines
Consider another non-right triangle, labeled as shown with side lengthsĀ xĀ andĀ y. We can derive a useful law containing only the cosine function.
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First use the Pythagorean theorem to derive two equations for each of the right triangles:
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Notice that each contains andĀ x2, so we can eliminateĀ x2Ā between the two using the transitive property:
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Then expand the binomialĀ (b - y)2Ā to get the equation below, and note that theĀ y2Ā cancel:
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Now we still have a y hanging around, but we can get rid of it using the cosine solution, notice that
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Substituting cĀ·cos(A) for y, we get
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which is the law of cosines
The law of cosines can be used to find the measure of an angle or a side of a non-right triangle if we know:
two sides and the angle between them orthree sides and no angles.
We could again do the same derivation using the other two altitudes of our triangle, to yield three versions of the law of cosines for any triangle. They are listed in the box below.
Law of Cosines
TheĀ Law of CosinesĀ is just the Pythagorean relationship with a correction factor, e.g.Ā -2bcĀ·cos(A), to account for the fact that the triangle is not a right triangle. We can write three versions of the LOC, one for every angle/opposite side pair:
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Examples: Law of cosines
Use the law of cosines to find the missing measurements of the triangles in these two examples. In the first, the measures of two sides and the included angle (the angle between them) are known. In the second, three sides are known.