三角函数是怎么运作的？

I believe they're calculated using Taylor Series or CORDIC. Some applications which make heavy use of trig functions (games, graphics) construct trig tables when they start up so they can just look up values rather than recalculating them over and over.

First, you have to do some sort of range reduction. Trig functions are periodic, so you need to reduce arguments down to a standard interval. For starters, you could reduce angles to be between 0 and 360 degrees. But by using a few identities, you realize you could get by with less. If you calculate sines and cosines for angles between 0 and 45 degrees, you can bootstrap your way to calculating all trig functions for all angles.

Once you've reduced your argument, most chips use a CORDIC algorithm to compute the sines and cosines. You may hear people say that computers use Taylor series. That sounds reasonable, but it's not true. The CORDIC algorithms are much better suited to efficient hardware implementation. (Software libraries may use Taylor series, say on hardware that doesn't support trig functions.) There may be some additional processing, using the CORDIC algorithm to get fairly good answers but then doing something else to improve accuracy.

There are some refinements to the above. For example, for very small angles theta (in radians), sin(theta) = theta to all the precision you have, so it's more efficient to simply return theta than to use some other algorithm. So in practice there is a lot of special case logic to squeeze out all the performance and accuracy possible. Chips with smaller markets may not go to as much optimization effort.

Check out the Wikipedia article on trig functions. A good place to learn about actually implementing them in code is Numerical Recipes.

I'm not much of a mathematician, but my understanding of where sin, cos, and tan "come from" is that they are, in a sense, observed when you're working with right-angle triangles. If you take measurements of the lengths of sides of a bunch of different right-angle triangles and plot the points on a graph, you can get sin, cos, and tan out of that. As Harper Shelby points out, the functions are simply defined as properties of right-angle triangles.

A more sophisticated understanding is achieved by understanding how these ratios relate to the geometry of circle, which leads to radians and all of that goodness. It's all there in the Wikipedia entry.

edit: Jack Ganssle has a decent discussion in his book on embedded systems, "The Firmware Handbook".

FYI: If you have accuracy and performance constraints, Taylor series should not be used to approximate functions for numerical purposes. (Save them for your Calculus courses.) They make use of the analyticity of a function at a single point, e.g. the fact that all its derivatives exist at that point. They don't necessarily converge in the interval of interest. Often they do a lousy job of distributing the function approximation's accuracy in order to be "perfect" right near the evaluation point; the error generally zooms upwards as you get away from it. And if you have a function with any noncontinuous derivative (e.g. square waves, triangle waves, and their integrals), a Taylor series will give you the wrong answer.

The best "easy" solution, when using a polynomial of maximum degree N to approximate a given function f(x) over an interval x0 < x < x1, is from Chebyshev approximation; see Numerical Recipes for a good discussion. Note that the Tj(x) and Tk(x) in the Wolfram article I linked to used the cos and inverse cosine, these are polynomials and in practice you use a recurrence formula to get the coefficients. Again, see Numerical Recipes.

edit: Wikipedia has a semi-decent article on approximation theory. One of the sources they cite (Hart, "Computer Approximations") is out of print (& used copies tend to be expensive) but goes into a lot of detail about stuff like this. (Jack Ganssle mentions this in issue 39 of his newsletter The Embedded Muse.)

edit 2: Here's some tangible error metrics (see below) for Taylor vs. Chebyshev for sin(x). Some important points to note:

1. that the maximum error of a Taylor series approximation over a given range, is much larger than the maximum error of a Chebyshev approximation of the same degree. (For about the same error, you can get away with one fewer term with Chebyshev, which means faster performance)
2. Range reduction is a huge win. This is because the contribution of higher order polynomials shrinks down when the interval of the approximation is smaller.
3. If you can't get away with range reduction, your coefficients need to be stored with more precision.

Don't get me wrong: Taylor series will work properly for sine/cosine (with reasonable precision for the range -pi/2 to +pi/2; technically, with enough terms, you can reach any desired precision for all real inputs, but try to calculate cos(100) using Taylor series and you can't do it unless you use arbitrary-precision arithmetic). If I were stuck on a desert island with a nonscientific calculator, and I needed to calculate sine and cosine, I would probably use Taylor series since the coefficients are easy to remember. But the real world applications for having to write your own sin() or cos() functions are rare enough that you'd be best off using an efficient implementation to reach a desired accuracy -- which the Taylor series is not.

Range = -pi/2 to +pi/2, degree 5 (3 terms)

• Taylor: max error around 4.5e-3, f(x) = x-x³/6+x⁵/120
• Chebyshev: max error around 7e-5, f(x) = 0.9996949x-0.1656700x³+0.0075134x⁵

Range = -pi/2 to +pi/2, degree 7 (4 terms)

• Taylor: max error around 1.5e-4, f(x) = x-x³/6+x⁵/120-x⁷/5040
• Chebyshev: max error around 6e-7, f(x) = 0.99999660x-0.16664824x³+0.00830629x⁵-0.00018363x⁷

Range = -pi/4 to +pi/4, degree 3 (2 terms)

• Taylor: max error around 2.5e-3, f(x) = x-x³/6
• Chebyshev: max error around 1.5e-4, f(x) = 0.999x-0.1603x³

Range = -pi/4 to +pi/4, degree 5 (3 terms)

• Taylor: max error around 3.5e-5, f(x) = x-x³/6+x⁵
• Chebyshev: max error around 6e-7, f(x) = 0.999995x-0.1666016x³+0.0081215x⁵

Range = -pi/4 to +pi/4, degree 7 (4 terms)

• Taylor: max error around 3e-7, f(x) = x-x³/6+x⁵/120-x⁷/5040
• Chebyshev: max error around 1.2e-9, f(x) = 0.999999986x-0.166666367x³+0.008331584x⁵-0.000194621x⁷

If your asking for a more physical explanation of sin, cos, and tan consider how they relate to right-angle triangles. The actual numeric value of cos(lambda) can be found by forming a right-angle triangle with one of the angles being lambda and dividing the length of the triangles side adjacent to lambda by the length of the hypotenuse. Similarily for sin use the opposite side divided by the hypotenuse. For tangent use the opposite side divided by the adjacent side. The classic memonic to remember this is SOHCAHTOA (pronounced socatoa).

Most commonly for computers, power series representation is used to calculate sines and cosines and these are used for other trig functions. Expanding these series out to about 8 terms computes the values needed to an accuracy close to the machine epsilon (smallest non-zero floating point number that can be held).

The CORDIC method is faster since it is implemented on hardware, but it is primarily used for embedded systems and not standard computers.

I would like to extend the answer provided by @Jason S. Using a domain subdivision method similar to that described by @Jason S and using Maclaurin series approximations, an average (2-3)X speedup over the tan(), sin(), cos(), atan(), asin(), and acos() functions built into the gcc compiler with -O3 optimization was achieved. The best Maclaurin series approximating functions described below achieved double precision accuracy.

For the tan(), sin(), and cos() functions, and for simplicity, an overlapping 0 to 2pi+pi/80 domain was divided into 81 equal intervals with "anchor points" at pi/80, 3pi/80, ..., 161pi/80. Then tan(), sin(), and cos() of these 81 anchor points were evaluated and stored. With the help of trig identities, a single Maclaurin series function was developed for each trig function. Any angle between ±infinity may be submitted to the trig approximating functions because the functions first translate the input angle to the 0 to 2pi domain. This translation overhead is included in the approximation overhead.

Similar methods were developed for the atan(), asin(), and acos() functions, where an overlapping -1.0 to 1.1 domain was divided into 21 equal intervals with anchor points at -19/20, -17/20, ..., 19/20, 21/20. Then only atan() of these 21 anchor points was stored. Again, with the help of inverse trig identities, a single Maclaurin series function was developed for the atan() function. Results of the atan() function were then used to approximate asin() and acos().

Since all inverse trig approximating functions are based on the atan() approximating function, any double-precision argument input value is allowed. However the argument input to the asin() and acos() approximating functions is truncated to the ±1 domain because any value outside it is meaningless.

To test the approximating functions, a billion random function evaluations were forced to be evaluated (that is, the -O3 optimizing compiler was not allowed to bypass evaluating something because some computed result would not be used.) To remove the bias of evaluating a billion random numbers and processing the results, the cost of a run without evaluating any trig or inverse trig function was performed first. This bias was then subtracted off each test to obtain a more representative approximation of actual function evaluation time.

Table 2. Time spent in seconds executing the indicated function or functions one billion times. The estimates are obtained by subtracting the time cost of evaluating one billion random numbers shown in the first row of Table 1 from the remaining rows in Table 1.

Time spent in tan(): 18.0515 18.2545

Time spent in TAN3(): 5.93853 6.02349

Time spent in TAN4(): 6.72216 6.99134

Time spent in sin() and cos(): 19.4052 19.4311

Time spent in SINCOS3(): 7.85564 7.92844

Time spent in SINCOS4(): 9.36672 9.57946

Time spent in atan(): 15.7160 15.6599

Time spent in ATAN1(): 6.47800 6.55230

Time spent in ATAN2(): 7.26730 7.24885

Time spent in ATAN3(): 8.15299 8.21284

Time spent in asin() and acos(): 36.8833 36.9496

Time spent in ASINCOS1(): 10.1655 9.78479

Time spent in ASINCOS2(): 10.6236 10.6000

Time spent in ASINCOS3(): 12.8430 12.0707

(In the interest of saving space, Table 1 is not shown.) Table 2 shows the results of two separate runs of a billion evaluations of each approximating function. The first column is the first run and the second column is the second run. The numbers '1', '2', '3' or '4' in the function names indicate the number of terms used in the Maclaurin series function to evaluate the particular trig or inverse trig approximation. SINCOS#() means that both sin and cos were evaluated at the same time. Likewise, ASINCOS#() means both asin and acos were evaluated at the same time. There is little extra overhead in evaluating both quantities at the same time.

The results show that increasing the number of terms slightly increases execution time as would be expected. Even the smallest number of terms gave around 12-14 digit accuracy everywhere except for the tan() approximation near where its value approaches ±infinity. One would expect even the tan() function to have problems there.

Similar results were obtained on a high-end MacBook Pro laptop in Unix and on a high-end desktop computer in Linux.