# Figure out the numerical values of sines and cosines?

1. Oct 20, 2003

### StephenPrivitera

How do mathematicians figure out the numerical values of sines and cosines? I can figure out how to evaluate sin(pi/12), sin(pi/24), sin(pi/48), etc, using sin(pi/6) and half angle formulas. How would I find sin(pi/5), for example? Is there any way other than infinite sums to express the value of a sine?

Edit: stupid mistake

Last edited by a moderator: Feb 6, 2013
2. Oct 20, 2003

### Hurkyl

Staff Emeritus
Use the 1/n-th angle formula!

Well, it's not quite that simple... but one way to get the value is to do something like the following:

cos 3&theta; = cos (2&theta; + &theta;)
= cos 2&theta; cos &theta; - sin 2&theta; sin &theta;
= (2 (cos &theta;)^2 - 1) cos &theta; - 2 (sin &theta;)^2 cos &theta;
= 2 (cos &theta;)^3 - cos &theta; - 2 cos &theta; + 2 (cos &theta;)^3
= 4 (cos &theta;)^3 - 3 cos &theta;

If you plug in &pi; / 9 for &theta;, you have a polynomial in cos &theta; that you can solve.

In general, though, one cannot write the value of sin (&pi; p/q) in terms of +, -, *, /, and roots.

When I derive the value for sin (&pi;/5), I use geometry. I draw a regular pentagon and all of its diagonals, and through some magic with similar triangles, I can get a simple equation I can solve.

3. Oct 20, 2003

### HallsofIvy

Staff Emeritus
Actually most mathematicians use calculators to figure out values of trig functions!

For most people, the simplest way to calculate (by "hand") the approximate values of is to use the "Taylor series".

For any x, sin(x)= x- (1/6)x3+ 1/(5!) x5- 1/(7!) x7+ ... + (-1)2n+1/(2n+1)! x2n+1 and
cos(y)= 1- 1/2 x2+ 1/4! x4- 1/6! x6+ ...+ (-1)2n/(2n)! x2n.
If x is reasonably small, you don't have to take n very large at all.
(x is in radians, of course.)

I used to think that that was how calculators and computers did trig functions but I have been told that they actually use a much more sophisticated set of algorithms. Unfortunately, I've forgotten what they are called!

4. Oct 20, 2003

### chroot

Staff Emeritus
Halls,

He asked for ways to calculate WITHOUT power series.

And most calculators use a very sophisticated method called "table lookup."

- Warren

5. Oct 20, 2003

### StephenPrivitera

That disappoints me. I guess I already knew that. It just seems so unlikely that something as simple as a circle can lead so directly to such complicated results.

6. Oct 22, 2003

### HallsofIvy

Staff Emeritus
In other words, sine and cosine are not "algebraic functions".

Actually, most functions are not algebraic (they are "transcendental functions").

7. Oct 22, 2003