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The Law of SINES

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The Law of SINES For any triangle (right, acute or obtuse), you may use the following formula to solve for missing sides or angles:

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**Use Law of SINES when ... AAS - 2 angles and 1 adjacent side**

you have 3 dimensions of a triangle and you need to find the other 3 dimensions - they cannot be just ANY 3 dimensions though, or you won’t have enough info to solve the Law of Sines equation. Use the Law of Sines if you are given: AAS - 2 angles and 1 adjacent side ASA - 2 angles and their included side SSA (this is an ambiguous case)

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Example 1 You are given a triangle, ABC, with angle A = 70°, angle B = 80° and side a = 12 cm. Find the measures of angle C and sides b and c. * In this section, angles are named with capital letters and the side opposite an angle is named with the same lower case letter .*

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**Example 1 (con’t) A C B 70° 80° a = 12 c b**

The angles in a ∆ total 180°, so angle C = 30°. Set up the Law of Sines to find side b:

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**Example 1 (con’t) A C B 70° 80° a = 12 c b = 12.6 30°**

Set up the Law of Sines to find side c:

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**Example 1 (solution) A C B 70° 80° a = 12 c = 6.4 b = 12.6 30°**

Angle C = 30° Side b = 12.6 cm Side c = 6.4 cm Note: We used the given values of A and a in both calculations. Your answer is more accurate if you do not used rounded values in calculations.

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Example 2 You are given a triangle, ABC, with angle C = 115°, angle B = 30° and side a = 30 cm. Find the measures of angle A and sides b and c.

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Example 2 (con’t) To solve for the missing sides or angles, we must have an angle and opposite side to set up the first equation. We MUST find angle A first because the only side given is side a. The angles in a ∆ total 180°, so angle A = 35°. A C B 115° 30° a = 30 c b

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**Example 2 (con’t) A C B 115° a = 30 c b**

30° a = 30 c b 35° Set up the Law of Sines to find side b:

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**Example 2 (con’t) A C B 115° a = 30 c b = 26.2**

30° a = 30 c b = 26.2 35° Set up the Law of Sines to find side c:

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**Example 2 (solution) Angle A = 35° Side b = 26.2 cm Side c = 47.4 cm A**

115° 30° a = 30 c = 47.4 b = 26.2 35° Angle A = 35° Side b = 26.2 cm Side c = 47.4 cm Note: Use the Law of Sines whenever you are given 2 angles and one side!

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**The Ambiguous Case (SSA)**

When given SSA (two sides and an angle that is NOT the included angle) , the situation is ambiguous. The dimensions may not form a triangle, or there may be 1 or 2 triangles with the given dimensions. We first go through a series of tests to determine how many (if any) solutions exist.

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**The Ambiguous Case (SSA)**

In the following examples, the given angle will always be angle A and the given sides will be sides a and b. If you are given a different set of variables, feel free to change them to simulate the steps provided here. ‘a’ - we don’t know what angle C is so we can’t draw side ‘a’ in the right position A B ? b C = ? c = ?

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**The Ambiguous Case (SSA)**

Situation I: Angle A is obtuse If angle A is obtuse there are TWO possibilities If a ≤ b, then a is too short to reach side c - a triangle with these dimensions is impossible. If a > b, then there is ONE triangle with these dimensions. A B ? a b C = ? c = ? A B ? a b C = ? c = ?

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**The Ambiguous Case (SSA)**

Situation I: Angle A is obtuse - EXAMPLE Given a triangle with angle A = 120°, side a = 22 cm and side b = 15 cm, find the other dimensions. Since a > b, these dimensions are possible. To find the missing dimensions, use the Law of Sines: A B a = 22 15 = b C c 120°

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**The Ambiguous Case (SSA)**

Situation I: Angle A is obtuse - EXAMPLE Angle C = 180° - 120° ° = 23.8° Use Law of Sines to find side c: A B a = 22 15 = b C c 120° 36.2° Solution: angle B = 36.2°, angle C = 23.8°, side c = 10.3 cm

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**The Ambiguous Case (SSA)**

Situation II: Angle A is acute If angle A is acute there are SEVERAL possibilities. Side ‘a’ may or may not be long enough to reach side ‘c’. We calculate the height of the altitude from angle C to side c to compare it with side a. A B ? b C = ? c = ? a

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**The Ambiguous Case (SSA)**

Situation II: Angle A is acute First, use SOH-CAH-TOA to find h: A B ? b C = ? c = ? a h Then, compare ‘h’ to sides a and b . . .

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**The Ambiguous Case (SSA)**

Situation II: Angle A is acute If a < h, then NO triangle exists with these dimensions. A B ? b C = ? c = ? a h

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**The Ambiguous Case (SSA)**

Situation II: Angle A is acute If h < a < b, then TWO triangles exist with these dimensions. A B b C c a h A B b C c a h If we open side ‘a’ to the outside of h, angle B is acute. If we open side ‘a’ to the inside of h, angle B is obtuse.

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**The Ambiguous Case (SSA)**

Situation II: Angle A is acute If h < b < a, then ONE triangle exists with these dimensions. Since side a is greater than side b, side a cannot open to the inside of h, it can only open to the outside, so there is only 1 triangle possible! A B b C c a h

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**The Ambiguous Case (SSA)**

Situation II: Angle A is acute If h = a, then ONE triangle exists with these dimensions. A B b C c a = h If a = h, then angle B must be a right angle and there is only one possible triangle with these dimensions.

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**The Ambiguous Case (SSA)**

Situation II: Angle A is acute - EXAMPLE 1 Given a triangle with angle A = 40°, side a = 12 cm and side b = 15 cm, find the other dimensions. Find the height: A B ? 15 = b C = ? c = ? a = 12 h 40° Since a > h, but a< b, there are 2 solutions and we must find BOTH.

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**The Ambiguous Case (SSA)**

Situation II: Angle A is acute - EXAMPLE 1 FIRST SOLUTION: Angle B is acute - this is the solution you get when you use the Law of Sines! A B 15 = b C c a = 12 h 40°

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**The Ambiguous Case (SSA)**

Situation II: Angle A is acute - EXAMPLE 1 SECOND SOLUTION: Angle B is obtuse - use the first solution to find this solution. In the second set of possible dimensions, angle B is obtuse, because side ‘a’ is the same in both solutions, the acute solution for angle B & the obtuse solution for angle B are supplementary. Angle B = ° = 126.5° a = 12 A B 15 = b C c 40° 1st ‘a’ 1st ‘B’

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**The Ambiguous Case (SSA)**

Situation II: Angle A is acute - EXAMPLE 1 SECOND SOLUTION: Angle B is obtuse Angle B = 126.5° Angle C = 180°- 40° ° = 13.5° a = 12 A B 15 = b C c 40° 126.5°

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**The Ambiguous Case (SSA)**

Situation II: Angle A is acute - EX. 1 (Summary) Angle B = 53.5° Angle C = 86.5° Side c = 18.6 Angle B = 126.5° Angle C = 13.5° Side c = 4.4 a = 12 A B 15 = b C c = 4.4 40° 126.5° 13.5° A B 15 = b C c = 18.6 a = 12 40° 53.5° 86.5°

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**The Ambiguous Case (SSA)**

Situation II: Angle A is acute - EXAMPLE 2 Given a triangle with angle A = 40°, side a = 12 cm and side b = 10 cm, find the other dimensions. A B ? 10 = b C = ? c = ? a = 12 h 40° Since a > b, and h is less than a, we know this triangle has just ONE possible solution - side ‘a’opens to the outside of h.

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**The Ambiguous Case (SSA)**

Situation II: Angle A is acute - EXAMPLE 2 Using the Law of Sines will give us the ONE possible solution: A B 10 = b C c a = 12 40°

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**The Ambiguous Case - Summary**

if angle A is acute find the height, h = b*sinA if angle A is obtuse if a < b no solution if a > b one solution if a < h no solution if h < a < b 2 solutions one with angle B acute, one with angle B obtuse if a > b > h 1 solution If a = h 1 solution angle B is right (Ex I) (Ex II-1) (Ex II-2)

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**The Law of Sines AAS ASA SSA (the ambiguous case)**

Use the Law of Sines to find the missing dimensions of a triangle when given any combination of these dimensions.

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**Additional Resources Web Links:**

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Section 4.2 – The Law of Sines. If none of the angles of a triangle is a right angle, the triangle is called oblique. An oblique triangle has either three.

Section 4.2 – The Law of Sines. If none of the angles of a triangle is a right angle, the triangle is called oblique. An oblique triangle has either three.

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