Multiplying Large Numbers Using Sine and Cosine
Date: 12/17/2002 at 14:31:58 From: Justin Subject: Multiplication using sine and cosine I am researching Tycho Brahe and have come upon an example where he uses [sin(a + b) + sin(a - b)]/2 to multiply large numbers together due to the availability of sine tables. The example given is: "If a person wanted to find the product of a problem such as 155 x 36, a person would find the angle values for the decimals of these numbers. Thus, sin 77°49' = 0.15500 and cos 68°54’ = 0.36000. Thus, 36 x 155 = 105 [sin 77°49' + sin (-59°59')] / 2 = (97748 - 86588) / 2 = 5580. Some significant figures would be lost but for large numbers this procedure saves labor." I am unable to come up with the same results or figure out how this works. Most of the confusion arises with my unfamiliarity with degrees and minutes and how I would use them in a calculation. I have tried going backward from 0.15500, etc... to no avail. I am also not sure how this would be a universal algorithm.
Date: 12/17/2002 at 16:46:54 From: Doctor Peterson Subject: Re: Multiplication using sine and cosine Hi, Justin. I found a page that describes this method in some detail: Henry Briggs - Introduction: the need for speed in calculation http://cerebro.cs.xu.edu/math/math147/02f/briggs/briggsintro.html The essential formula given there is cos(a)cos(b) = [cos(a + b) + cos(a - b)]/2 (Your version uses sines, but the work will be similar. You should be able to prove both formulas using the angle-sum identities.) This, like logarithms, converts a multiplication on the left to an addition on the right. Since cosines are never greater than one, you have to move the decimal point in the given numbers while you do the work, and then insert the decimal point at the right place in the end, just as when you multiply decimals by hand. To multiply 155 by 36, we will be actually using the formula to multiply 0.155 by 0.36. We use the inverse cosine (that is, look up the angle whose cosine in the table is as close as possible to each of these numbers) to find cos(a) = 0.155 ==> a = 81.0832° cos(b) = 0.36 ==> b = 68.8998° Note that you don't HAVE to use minutes and seconds if you don't want to; that is not an essential part of this process, but arose from the fact that their trig tables used them. I used my calculator and got decimal degrees; or I could use radians just as well. Now what will the right side of the equation look like with these values? cos(a+b) = cos(149.9830°) = -0.865877 cos(a-b) = cos(12.1834°) = 0.977477 Averaging these, we find that the product of 0.155 and 0.36 is (-0.865877 + 0.977477)/2 = 0.1116/2 = 0.0558 To convert this back to 155 * 36, we have to multiply by 1000*100, moving the decimal point right 5 places; so the answer is 5580. This is correct; we found it by a few table lookups, a few additions and subtractions, and a division by 2. Back in a time when people were afraid to depend on memorized multiplication tables, that was a big savings, especially for larger numbers. Now, suppose you did have to use degrees and minutes. The only place where you have to do arithmetic on them is when you add and subtract the angles. We have a = cos^-1(0.155) = 81° 4' 59" b = cos^-1(0.36) = 68° 53' 59" according to my calculator. To add them, just add the parts, first: a+b = (81+68)° (4+53)' (59+59)" = 149° 57' 118" Everything is fine except the seconds, which should be less than 60; so we change 60 seconds to 1 minute and have 118" = 60" + 58" = 1' 58" so a+b = 149° 58' 58" You do the same sort of thing to subtract: a-b = (81-68)° (4-53)' (59-59)" We can't have a negative number of minutes, so we "borrow" a degree and add 60 minutes: a-b = (13-1)° (60+4-53)' 0" = 12° 11' Now you take the cosine of these, so there's no more to do with minutes. All this carrying and borrowing should look familiar from elementary arithmetic; we're actually working in base 60 (sexagesimal), a leftover from the ancient Babylonian number system! If you have any further questions, feel free to write back. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/
Date: 12/17/2002 at 18:34:53 From: Justin Subject: Thank you (Multiplication using sine and cosine) Thank you very much for you help. Your explanation was very thorough and will definitely help me put the finishing touches on my research paper. Justin
Search the Dr. Math Library:
Ask Dr. MathTM
© 1994- The Math Forum at NCTM. All rights reserved.