This seventeenth article of the mathematical journey through open source, takes through a tour of the high school trigonometry using Maxima.

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Trigonometry first gets introduced to students of standard IX, through triangles. And, then it takes its own journey through the jungle of formulae and tables. And one knows, being good at instant recall of various formulae, makes her good at trigo. The idea here is not to be good at mugging up the formulae but rather applying them to get the various end results. It assumes that you possibly already know the formulae.

Fundamental trigonometric functions

Maxima provides all the familiar fundamental trigonometric functions, including the hyperbolic ones. They can be tabulated as follows:

Mathematical Names	Normal		Hyperbolic
	Functions	Inv. Functions	Functions	Inv. Functions
sine (sin)	sin()	asin()	sinh()	asinh()
cosine (cos)	cos()	acos()	cosh()	acosh()
tangent (tan)	tan()	atan()	tanh()	atanh()
cosecant (cosec)	csc()	acsc()	csch()	acsch()
secant (sec)	sec()	asec()	sech()	asech()
cotangent (cot)	cot()	acot()	coth()	acoth()

Note that all of their arguments are values in radians. And here follows a demonstration of a small subset of those:

$ maxima -q
(%i1) cos(0);
(%o1)                                  1
(%i2) cos(%pi/2);
(%o2)                                  0
(%i3) cot(0);
The number 0 isn't in the domain of cot
 -- an error. To debug this try: debugmode(true);
(%i4) tan(%pi/4);
(%o4)                                  1
(%i5) string(asin(1));
(%o5)                                %pi/2
(%i6) csch(0);
The number 0 isn't in the domain of csch
 -- an error. To debug this try: debugmode(true);
(%i7) csch(1);
(%o7)                               csch(1)
(%i8) asinh(0);
(%o8)                                  0
(%i9) string(%i * sin(%pi / 3)^2 + cos(5 * %pi / 6));
(%o9)                         3*%i/4-sqrt(3)/2
(%i10) quit();

Simplifications with special angles like %pi / 10 and its multiples could be enabled by loading the ntrig package. Check out the difference below, before and after loading the package:

$ maxima -q
(%i1) string(sin(%pi/10));
(%o1)                             sin(%pi/10)
(%i2) string(cos(2*%pi/10));
(%o2)                             cos(%pi/5)
(%i3) string(tan(3*%pi/10));
(%o3)                            tan(3*%pi/10)
(%i4) load(ntrig);
(%o4)        /usr/share/maxima/5.24.0/share/trigonometry/ntrig.mac
(%i5) string(sin(%pi/10));
(%o5)                            (sqrt(5)-1)/4
(%i6) string(cos(2*%pi/10));
(%o6)                            (sqrt(5)+1)/4
(%i7) string(tan(3*%pi/10));
(%o7)          sqrt(2)*(sqrt(5)+1)/((sqrt(5)-1)*sqrt(sqrt(5)+5))
(%i8) quit();

A very common trigonometric problem is, given a tangent value find the corresponding angle. Now, a common challenge is for every value, the angle could lie in two quadrants. For a positive tangent, the angle could be in the first or the third quadrant, and for a negative value, the angle could be in the second or the fourth quadrant. So, atan() cannot always calculate the correct quadrant of the angle. Now, how do we know it exactly. Obviously, we need some extra information, say the actual values of the perpendicular (p) and the base (b) of the tangent, rather than just the tangent value. With that, the angle location could be tabulated as follows:

Perpendicular (p)	Base (b)	Tangent (p/b)	Angle Quadrant
Positive	Positive	Positive	First
Positive	Negative	Negative	Second
Negative	Negative	Positive	Third
Negative	Positive	Negative	Fourth

And this functionality is captured in the atan2() function, which takes 2 arguments, the p and the b, and thus does provide the angle in the correct quadrant, as per the table above. Along with this, the infinities of tangent are also taken care. Here’s a demo:

$ maxima -q
(%i1) atan2(0, 1); /* Zero */
(%o1)                                  0
(%i2) atan2(0, -1); /* Zero */
(%o2)                                 %pi
(%i3) string(atan2(1, -1)); /* -1 */
(%o3)                               3*%pi/4
(%i4) string(atan2(-1, -1)); /* 1 */
(%o4)                              -3*%pi/4
(%i5) string(atan2(-1, 0)); /* - Infinity */
(%o5)                               -%pi/2
(%i6) string(atan2(5, 0)); /* + Infinity */
(%o6)                                %pi/2
(%i7) quit();

Trigonometric Identities

Maxima supports many built-in trigonometric identities, and one can add his own as well. First one to look at would be the set dealing with integral multiples and factors of %pi. We’ll declare a few integers and then play around with them.

$ maxima -q
(%i1) declare(m, integer, n, integer);
(%o1)                                done
(%i2) properties(m);
(%o2)                  [database info, kind(m, integer)]
(%i3) sin(m * %pi);
(%o3)                                  0
(%i4) string(cos(n * %pi));
(%o4)                              (-1)^n
(%i5) string(cos(m * %pi / 2)); /* No simplification */
(%o5)                            cos(%pi*m/2)
(%i6) declare(m, even); /* Will lead to simplification */
(%o6)                               done
(%i7) declare(n, odd);
(%o7)                               done
(%i8) cos(m * %pi);
(%o8)                                 1
(%i9) cos(n * %pi);
(%o9)                                - 1
(%i10) string(cos(m * %pi / 2));
(%o10)                             (-1)^(m/2)
(%i11) string(cos(n * %pi / 2));
(%o11)                            cos(%pi*n/2)
(%i12) quit();

Next is the relation between the normal & the hyperbolic trigo functions.

$ maxima -q
(%i1) sin(%i * x);
(%o1)                            %i sinh(x)
(%i2) cos(%i * x);
(%o2)                              cosh(x)
(%i3) tan(%i * x);
(%o3)                            %i tanh(x)
(%i4) quit();

By enabling the option variable halfangles, many half angle identities, come into play. To be specific, sin(x/2) gets further simplified in (0, 2 * %pi) range, cos(x/2) gets further simplified in (-%pi/2, %pi/2) range. Check out the differences, before and after enabling the option variable, along with the range modifications, in the examples below:

$ maxima -q
(%i1) string(2*cos(x/2)^2 - 1); /* No effect */
(%o1)                           2*cos(x/2)^2-1
(%i2) string(cos(x/2)); /* No effect */
(%o2)                              cos(x/2)
(%i3) halfangles:true; /* Enabling half angles */
(%o3)                                true
(%i4) string(2*cos(x/2)^2 - 1); /* Simplified */
(%o4)                               cos(x)
(%i5) string(cos(x/2)); /* Complex expansion for all x */
(%o5)         (-1)^floor((x+%pi)/(2*%pi))*sqrt(cos(x)+1)/sqrt(2)
(%i6) assume(-%pi < x, x < %pi); /* Limiting x values */
(%o6)                        [x > - %pi, x < %pi]
(%i7) string(cos(x/2)); /* Further simplified */
(%o7)                       sqrt(cos(x)+1)/sqrt(2)
(%i8) quit();

Trigonometric Expansions and Simplifications

Trigonometry is full of multiples of angles, sums of angles, products & powers of trigo functions, and the long list of relations between them. Multiples and sums of angles fall into one category. Products and powers of trigo functions in an another category. And its very useful to do conversions from one of these categories to the other one, to crack a range of simple to complex problems, catering to basic hobby science to quantum mechanics. trigexpand() does the conversion from “multiples & sums of angles” to “products & powers of trigo functions”. trigreduce() does exactly the opposite. Here’s goes a small demo:

$ maxima -q
(%i1) trigexpand(sin(2*x));
(%o1)                           2 cos(x) sin(x)
(%i2) trigexpand(sin(x+y)-sin(x-y));
(%o2)                           2 cos(x) sin(y)
(%i3) trigexpand(cos(2*x+y)-cos(2*x-y));
(%o3)                         - 2 sin(2 x) sin(y)
(%i4) trigexpand(%o3);
(%o4)                      - 4 cos(x) sin(x) sin(y)
(%i5) string(trigreduce(%o4));
(%o5)                  -2*(cos(y-2*x)/2-cos(y+2*x)/2)
(%i6) string(trigsimp(%o5));
(%o6)                       cos(y+2*x)-cos(y-2*x)
(%i7) string(trigexpand(cos(2*x)));
(%o7)                         cos(x)^2-sin(x)^2
(%i8) string(trigexpand(cos(2*x) + 2*sin(x)^2));
(%o8)                         sin(x)^2+cos(x)^2
(%i9) trigsimp(trigexpand(cos(2*x) + 2*sin(x)^2));
(%o9)                                 1
(%i10) quit();

In %o5 above, you might have noted that the 2’s could have been cancelled for further simplification. But that is not the job of trigreduce(), and for that we have to apply the trigsimp() function as shown in %i6. In fact, many other trigonometric identities based simplification are achieved using trigsimp(). Check out the %i7 to %o9 sequences for another such example.

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