# Can you prove this?

Discussion in 'Math' started by steveb, Nov 24, 2008.

1. ### steveb Thread Starter Senior Member

Jul 3, 2008
2,433
469
I was wondering if anyone is able to either prove or disprove the following identity. Please see attached PDF file.

This identity came up in my work recently. I won't go into the details of the application, but I did two different approximate derivations of a function I needed in an engineering application. The two separate functions appeared to give the same numerical answer, but look quite different. One funtion was very simple (y=0.5*x) and the other is much more complicated with an arctan function (see attachment).

My first assumption was that these are probably exactly equal functions; however, when I numerically evaluate the error using double precision calculations in Matlab, I find the error near x=pi/3 to be considerable more than what I expect with double precision. The error is much less as x moves away from this value. Now this error could be due to round off error accumulation, but I'm not convinced of that. (Actually, it probably is due to a zero/zero condition)

I wanted to either prove or disprove this identity, but I've been too busy and can't justify doing this as part of my work since it's just a point of curiousity.

I thought someone here might enjoy tackling this one. I suspect the identity is true, and there is probably a simple elegant way to prove it. I thought of doing out a Taylor expansion, but that looks tedious.

• ###### Prove.pdf
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Last edited: Nov 24, 2008
2. ### mik3 Senior Member

Feb 4, 2008
4,846
63
See the attachment, its just application of trigonometric identities.

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3. ### steveb Thread Starter Senior Member

Jul 3, 2008
2,433
469
Not just trig identities... Also, a sharp mind!

Great work! I appreciate it very much.

4. ### Ratch New Member

Mar 20, 2007
1,068
3
steveb,

Another way is to convert the numerator into all sine terms and the demonminator into all cosine terms. Then apply the difference of sines identity to the numerator and the sum of cosines to the denominator. Everything will cancel except tan(x/2), which is what you are looking for.

Ratch

5. ### steveb Thread Starter Senior Member

Jul 3, 2008
2,433
469
Yes, that's good too. Thanks.

For me the stumbling block was thinking to represent the constants ${{\sqrt3} \over {2}}$ and 1/2 as cos and sin of a constant. So simple in hindsight, but I just missed it.

I appreciate the help though. Saves me some time, which is very short lately.

Last edited: Nov 26, 2008
6. ### boff1 Member

Oct 15, 2008
26
0
Clever solution!