Homework Help: Fairly basic trigonometric equation

1. Jan 4, 2008

intangible

Acos(A*x)-Bcos(B*x)=0, where A>B

Is there a general solution for an equation of this form?

Thanks,
intangible

2. Jan 4, 2008

Gib Z

I'm quite inclined to say there are no solutions for that :(

3. Jan 5, 2008

Feldoh

Acos(A*x) = Bcos(B*x) -- So we conclude that A = B

4. Jan 6, 2008

Kaan

How about A=Pi/2

B=3Pi/2

5. Jan 6, 2008

Gib Z

I really can not be bothered to expand cos (3x), but if you are, I will verify the rest. I highly doubt it works though.

6. Jan 6, 2008

intangible

Thanks to everyone participating, I think I got sorted it out by inspecting a similar behaving sine and its infinite product.

$$\sin x = x \prod_{n = 1}^\infty\left(1 - \frac{x^2}{\pi^2 n^2}\right)$$

If we assume Asin(pi*A*x)=Bsin(pi*B*x) we may easily inspect the existence of trivial roots if there are any. It turns out there are: for every natural number A and B (A>B) there is a trivial root at x=n. Also, if there is a natural number D so that it divides both A and B, then there will be the trivial roots x=n/D.

$$A \prod_{n=1}^\infty\left (A x - n)(A x + n)\right = B \prod_{n=1}^\infty\left (B x - n)(B x + n)\right$$

That leaves A-1 unknown roots per cycle if the former is the case, (A-B)/D per cycle for the latter. Unfortunately, the other roots clearly are irrational, perhaps even transcendent.

Regards,
intangible

(How do I mark this thread solved?)

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