Prove Law of Sines, Acute
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The questions below concern the following proof.
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).

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 |
A.
What is the missing reason in step 1?
- Given
- All acute triangles have altitudes from each vertex that intersect the opposite side
- Perpendicular Bisector Theorem (for triangles)
- All acute triangles have an orthocenter that lies within the triangle
B.
What is the missing statement in step 5?
- sin(A)=BDAB
- sin(A)=BDAD
- sin(A)=BCAC
- sin(A)=ADAB
C.
What is the missing statement in step 7?
- sin(C)=BDAB
- sin(C)=CSBC
- sin(C)=BCAC
- sin(C)=BDBC
D.
What is the missing statement in step 9?
- Transitive Property of Equality
- Law of sines
- Multiplication Property of Equality
- Trigonometric Identity
E.
What is the missing reason in step 12?
- Each of the three altitudes of an acute triangle intersects the side opposite the vertex through which the altitude passes
- The orthocenter of all acute triangles lies within the triangle
- Perpendicular Bisector Theorem (for triangles)
- All triangles have three altitudes
F.
What is the missing statement in step 16?
- sin(C)=ABAC
- sin(C)=CEAC
- sin(C)=AEAC
- sin(C)=AECE
G.
What is the missing statement in step 18?
- sin(B)=CDBC
- sin(B)=BEAB
- sin(B)=ABAC
- sin(B)=AEAB
H.
What is the missing statement in step 20?
- ABsin(B)=ACsin(C)
- AE=AE
- sin(C)=sin(B)
- AC=AB
I.
What is the missing reason in step 23?
- Previous result
- Transitive Property of Equality
- Multiplication Property of Equality
- Law of sines
J.
The proof given applies to which type of triangles?
- All triangles.
- Acute triangles.
- Non-right triangles.
- Scalene triangles.