How to Solve Triangles on TI-83/84
Copyright © 2015–2020 by Stan Brown
Copyright © 2015–2020 by Stan Brown
The program figures out whether you’re in degree or radian mode, and adjusts itself accordingly. The program doesn’t check for negative angles or two entered angles adding to more than 180° or 2π, so don’t be silly, m’kay?
The program works on all TI-83 Plus models, and all TI-84 models including the color models.
See also: For the computations, please see Solving Triangles. For the program code, see TRIANGLE.pdf in the accompanying TRIANGLE.zip file.
There are three methods to get the program into your calculator:
ENTER]. Then on hers press [
3], select TRIANGLE, and finally press [
ENTER]. If you get a prompt about a duplicate program, choose Overwrite.
It’s customary to refer to the angles as A, B, and C, and the sides as a, b, and c, such that side a is opposite angle A, side b is opposite angle B, and side c is opposite angle C.
When you run the TRIANGLE program, it prompts you to say which facts you know:
Each case has either one unique solution, or none. The exception is SSA, which could have zero, one, or two solutions depending on the numbers. For about that, see Example 3 below, and Special Note: Side-Side-Angle in Trig without Tears.
Given the three sides of the triangle
(menu item 5), you can use the
program to find the
three angles. Run the TRIANGLE program and select
5:SSS. Enter the three sides, and the program gives you
the area, the three angles, and the three sides. The first angle is
opposite the first side, the second angle opposite the second side, and
the third angle opposite the third side.
Angles are always displayed in degrees, with one decimal
place on black-white screens, two decimal places on color screens.
If one of the angles is ≥100°, the b&w
display may be too narrow. In that case, just press [
B], or [
C] to display the angle. It’s
the unrounded value, so you can also do this to get full precision. In
the screen shot at right, angle C is shown.
In this case, of course you already know the sides. But in
cases where the program is computing them, you might want more
precision than the program displays. Press [
E], or [
F] for sides a, b, c, or
G] for the area.
When you know two angles and a side not between them,
the known side a; then the angle opposite it is A and the angle
between them is B. Here a = 180, A = 31°, and B =
The third angle is 107°. The base of the triangle is 334.22; and the third side, opposite the 42° angle, is 233.85.
Suppose you know that two sides measure 8 and 10 units. You don’t know the angle between them, but you know that the angle opposite the 8-unit side is 45°. There are actually two triangles that meet these conditions, a larger triangle with B as an acute angle (<90°), and a smaller triangle with B as an obtuse angle (>90°). Those triangles are also shown separately here, in brown and blue respectively:
See Special Note: Side-Side-Angle for the exact conditions when SSA can give you zero, one, or two solutions.
Fortunately, the TRIANGLE program
has these conditions programmed in for you:
When there are two
possible solutions, the program prompts you to choose one. Here
I’ve chosen the acute angle for B, which gives the larger
And here I’ve chosen the obtuse angle for B, which gives the smaller triangle.
If you know any two angles, you know the third one and therefore you know the shape of the triangle. Then, any one side or the area will let you solve the triangle completely.
Example: Suppose you know that a triangle has angles of 30°, 50°, with area of 27.16. Make a sketch, using (spoiler alert!) 100° for the third angle and about 12 units for the long side.
6:AA and Area, and
when prompted enter the two angles and the area. (If you know all
three angles, it doesn’t matter which two you enter.)
Remember that angle A is opposite side a, so side a = 6 units is opposite angle A = 30°, side b = 9.19 units is opposite angle B = 50°, and side c = 11.82 units is opposite angle C = 100°.
The TRIANGLE program uses several variables. They’re listed here because you might have occasion to use some of them after the program finishes:
Updates and new info: https://BrownMath.com/ti83/