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The Law

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  1. The Law of SINES

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

  3. Use Law of SINES when ... • AAS - 2 angles and 1 adjacent side • ASA - 2 angles and their included side • SSA(this is an ambiguous case) 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:

  4. 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 .*

  5. B 80° a = 12 c 70° A C b Example 1 (AAS) The angles in a ∆ total 180°, so angle C = 30°. Set up the Law of Sines to find side b:

  6. B 80° a = 12 c 70° 30° A C b = 12.6 Example 1 (AAS) Set up the Law of Sines to find side c:

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

  8. 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.

  9. B 30° c a = 30 115° C A b Example 2 (ASA) 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°.

  10. B 30° c a = 30 115° C A 35° b Example 2 (ASA) Set up the Law of Sines to find side b:

  11. B 30° c a = 30 115° C A 35° b = 26.2 Example 2 (ASA) Set up the Law of Sines to find side c:

  12. B 30° c = 47.4 a = 30 115° C A 35° b = 26.2 Example 2 (solution) 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!

  13. 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.

  14. C = ? ‘a’ - we don’t know what angle C is so we can’t draw side ‘a’ in the right position b A B ? c = ? 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.

  15. C = ? C = ? a a b b A B ? A B ? c = ? c = ? The Ambiguous Case (SSA) Situation I: Angle A is obtuse If angle A is obtuse, then side a MUST be the longest side. If a ≤ b, then a is too short to reach side c – this triangle is impossible. If a > b, then there is ONE triangle with these dimensions.

  16. C a = 22 15 = b 120° A B c 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:

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

  18. C = ? a b h A B ? c = ? The Ambiguous Case (SSA) Situation II: Angle A is acute If angle A is acute there are SEVERAL possibilities. If a < h, then NO triangle exists with these dimensions. Use SOH-CAH-TOA to findh: Then compare a to h to ensure you have a triangle. If not, a solution is impossible.

  19. C C b b a h a h A B A c c B The Ambiguous Case (SSA) Situation II: Angle A is acute Once you have confirmed a triangle exists, compare a to b. If h < a < b, then TWO triangles are possible. If we open side ‘a’ to the inside of h, angle B is obtuse. If we open side ‘a’ to the outside of h, angle B is acute.

  20. C b a h A B c The Ambiguous Case (SSA) Situation II: Angle A is acute If a>b, then ONE triangle is possible. 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!

  21. C b a = h A c B The Ambiguous Case (SSA) Situation II: Angle A is acute Special Case: If a= h, then you have a right triangle If a = h, then angle B must be a right angle and there is only one possible triangle with these dimensions.

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

  23. C a = 12 15 = b h 40° A B c 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!

  24. C 1st ‘a’ 15 = b a = 12 40° A c B 1st ‘B’ 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 = 180 - 53.5° = 126.5°

  25. Angle B = 126.5° Angle C = 180°- 40°- 126.5° = 13.5° C 15 = b a = 12 126.5° 40° A B c The Ambiguous Case (SSA) Situation II: Angle A is acute - EXAMPLE 1 SECOND SOLUTION: Angle B is obtuse

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

  27. C = ? a = 12 10 = b h 40° A B ? c = ? 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. 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.

  28. C a = 12 10 = b 40° A B c The Ambiguous Case (SSA) Situation II: Angle A is acute - EXAMPLE 2 Using the Law of Sines will give us the ONE possible solution:

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

  30. 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. The Law of Sines

  31. Additional Resources • http://www.regentsprep.org/regents/math/algtrig/ATT12/lawofsines.htm • http://oakroadsystems.com/twt/solving.htm#SineLaw • http://oakroadsystems.com/twt/solving.htm#Detective • Web Links: