Trigonometry Question
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Let ΔABC be an acute triangle. Let A represent m∠A, B represent m∠B, and C represent m∠C. Prove the law of sines, BCsin(A)=ACsin(B)=ABsin(C).

Statement | Reason |
1.Construct altitude ¯BD such that D lies on ¯AC | 1. |
2.¯BD⊥¯AC | 2.Definition of an altitude |
3.∠ADB, ∠BDC are right angles | 3.Definition of perpendicular lines |
4.ΔADB, ΔBDC are right triangles | 4.Definition of right triangles |
5. | 5.Sine ratio in a right triangle |
6.ABsin(A)=BD | 6.Multiplication Property of Equality |
7. | 7.Sine ratio in a right triangle |
8.BCsin(C)=BD | 8.Multiplication Property of Equality |
9.ABsin(A)=BCsin(C) | 9. |
10.AB=BCsin(C)sin(A) | 10.Division Property of Equality |
11.ABsin(C)=BCsin(A) | 11.Division Property of Equality |
12.Construct altitude ¯AE such that E lies on ¯BC | 12. |
13.¯AE⊥¯BC | 13.Definition of an altitude |
14.∠AEB, ∠AEC are right angles | 14.Definition of perpendicular lines |
15.ΔAEB, ΔAEC are right triangles | 15.Definition of right triangles |
16. | 16.Sine ratio in a right triangle |
17.ACsin(C)=AE | 17.Multiplication Property of Equality |
18. | 18.Sine ratio in a right triangle |
19.ABsin(B)=AE | 19.Multiplication Property of Equality |
20. | 20.Transitive Property of Equality |
21.ABsin(B)sin(C)=AC | 21.Division Property of Equality |
22.ABsin(C)=ACsin(B) | 22.Division Property of Equality |
23.BCsin(A)=ACsin(B) | 23. |
24.BCsin(A)=ACsin(B)=ABsin(C) | 24.Combined results |
Grade 11 Trigonometry CCSS: HSG-SRT.D.10
- All triangles.
- Acute triangles.
- Non-right triangles.
- Scalene triangles.