# Law of sines

1. Oct 1, 2008

### 838

I got into an argument with a mathematics grad student about the law of sines. He said that he has proven it to be wrong several times. I don't see how this is possible, given that the angles are opposing the sides, it doesn't seem possible for it to be EVER incorrect.

I asked him to prove it for me but he said he didn't have time.

In my years of studying engineering, I have NEVER found the law of sines to give me a false answer.

2. Oct 1, 2008

### robert Ihnot

If it was wrong, why would it be in all the books? Maybe he means sperical geometry, or something else.

3. Oct 1, 2008

### grmnsplx

as robert said, the grad student may have been talking about some non-Eulidean geometry.

If we take away one of Euclid's axioms, we make all new geometries and some of our inferences such as the law of sines may disappear.

Anyway, ask him about it again and put it on the forum.

4. Oct 1, 2008

### HallsofIvy

Some grad students are fools, some are just wise-acres.

Many like to put undergraduates on.

Yes, in non-Euclidean geometry, the "sin law" is not true. If you look at your text books, you will see that they never claimed it to be true except in Euclidean geometry.

If your grad student actually thought he had proven the "sin law" to be false in Euclidean geometry, he is a fool. If he meant "in non-Euclidean" geometry, but did not specify that, he is a wise-acre. I don't recommend hanging around with either sort.