How can I prove the sine theorem?

In summary, the conversation discusses the proof of the law of sines in a triangle ABC, with the use of cross products and inscribing the triangle in a circle. The conversation also mentions finding a solution on Wikipedia.
  • #1
rado5
73
0

Homework Statement



How can I prove this theorem in the triangle ABC? sin(A)/a=sin(B)/b=sin(C)/c



Homework Equations



A*B and ...

The Attempt at a Solution



I have drawn a triangle and tried to prove it, but i couldn't. (I don't know how to send a picture to my post!) I know how to prove the cosine theorem but i can't prove the sine theorem.
 
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  • #2

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  • #3
Consider the three vectors A to B, B to C and C to A. Now consider geometrically their cross products. The magnitude of each is equal to twice the area of the triangle, so the three are equal.
 
  • #4
I particularly like this one:

For any triangle, inscribe it in a circle. So the triangle touches the circle at 3 points, call them A, B, and C, and their corresponding angles a, b, and c, and the lengths of the sides opposite them L(a), L(b), L(c).

Move point A along the circumference so that AB passes the center of the circle. Angles subtended by equal chords onto the circumference are equal, so angle BAC is still a.

Now we have a triangle in a semi circle, and so angle ACB is equal to 90 degrees. By trigonometry, sin a = L(a)/Diameter, and so Diameter = sin a/L(a)

Making similar transformations for the other sides shows sin b/L(b) and sin c/L(c) are also equal to the diameter, and hence equal to each other.
 
  • #5
Oh, thank you very much really.
 

FAQ: How can I prove the sine theorem?

What is the sine theorem and why is it important?

The sine theorem, also known as the law of sines, is a fundamental theorem in trigonometry that relates the sides and angles of a triangle. It states that the ratio of a side of a triangle to the sine of the opposite angle is constant for all three sides of the triangle. This theorem is important because it allows us to solve for unknown sides and angles in triangles, which has many applications in mathematics, physics, and engineering.

How can I prove the sine theorem?

To prove the sine theorem, you can use the Law of Cosines or the Pythagorean Theorem. First, use the Law of Cosines to derive a formula for the sine of an angle in terms of the sides of the triangle. Then, use the Pythagorean Theorem to show that the ratio of a side to the sine of the opposite angle is constant for all three sides of the triangle. This will prove the sine theorem.

Can the sine theorem be used for all types of triangles?

Yes, the sine theorem can be used for all types of triangles, including right triangles, acute triangles, and obtuse triangles. However, the triangle must have at least one known angle and its opposite side in order for the theorem to be applied.

Is the sine theorem the only way to solve for unknown sides and angles in triangles?

No, the sine theorem is not the only way to solve for unknown sides and angles in triangles. Other methods include the Law of Cosines, the Law of Tangents, and the Pythagorean Theorem. The choice of method depends on the information given and the type of triangle.

Can the sine theorem be applied to non-right triangles?

Yes, the sine theorem can be applied to non-right triangles. In fact, the sine theorem is most commonly used for non-right triangles. This is because the other methods for solving triangles, such as the Pythagorean Theorem and the Law of Cosines, are specifically designed for right triangles.

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