1. Oblique triangles
a. Formulae
An oblique triangle is one in which individual angles are greater than 0° and less than 180°. A right triangle is a special oblique triangle in which one angle is exactly 90°.
A plane triangle has six parts: three angles and three sides.
Figure B1 Triangle Parts 
Equations for triangle trigonometry are:
Angle Condition  Equation B1  
Law of Sines  Equation B2  
Law of Cosines  Equation B3 
A right triangle is a special case where one angle is exactly 90°. If we apply the Law of Cosines, Equation B3, to a right triangle:
The Law of Cosines becomes the Pythagorean Theorem. 

Figure B2 Right triangle 
b. Solving triangles
In order to geometrically fix a triangle you must start with at least three parts and one of those must be a side. Why a side? Because fixing the angles alone does not constrain the size of the triangle.
These two triangles have identical angles but are different sizes.  
Figure B2 Angles Don't Fix Triangle 
Starting with three parts of a triangle, there is only one way to begin solving for another of its parts. Once a fourth part is determined, there are more ways to solve the remaing parts.
For example:
Given three sides of a triangle, how would you begin solving for other parts of it?  
Start with the Law of Cosines to solve for one angle. a^{2}=b^{2}+c^{2}2x(bc)xCos(A) Can then use Law of Cosines or Law of Sines to solve for next angle. Can then use Law of Cosines or Law of Sines to solve for next angle. Last angle can be solved by Law of Cosines, Law of Sines, or Angle Condition 
How about two sides and an included angle?  
Start with the Law of Cosines to solve for side a (known values in red): a^{2}=b^{2}+c^{2}2x(bc)xCos(A) Can then use Law of Cosines or Law of Sines to solve for another angle. Last angle can be solved by Law of Cosines, Law of Sines, or Angle Condition 
How would you solve these triangles?
Two angles and the included side.  
Two angles an a nonincluded side. 

Two sides and a nonincluded angle. 
c. Area
Another attribute of a triangle is its area which can be determined by one of two equations:
Using two sides and an included angle.  Equation B4  
Using all three sides  Heron's Formula.  Equation B5 
Given two parts of a triangle and its area, you should be able to determine its other parts, right?
Starting with two sides and an area...  
Solve angle A using the area equation: Area = 1/2(bc)xCos (A) Then use Law of Cosines to solve side a a^{2}=b^{2}+c^{2}2(bc)Cos(A) Then..... 

How about starting with an angle, opposite side, and triangle area? 
2. Trigonometric Functions
a. Sine; Law of Sines
The sine of any angle falls between 1.0 and +1.0. Refer to the sine curve plot in Figure B4 to see how angles and their sines relate. This curve repeats itself every 360°;
so sin(340°) = sin(20°) = sin(380°)...
Figure B4 
Taking the arcsine (a.k.a. inverse sine, sin^{1}) of a number between 1.0 and +1.0 on a calculator will always return an angle between 90° and +90°.
For example:
Sin(101°) is 0.981627
Sin^{1}(0.866025) is 79°
We started with 101° and wound up with 79°.
Sly obersverse that we are, we notice that 101°+79° is 180°
Table B1 show some other angles, their sine behavior, relationship between the beginnng and ending angles.
Table B1  
α  x=sin (α)  sin^{1}(x)  relationship 
30°  +0.50000  +30°  α 
150°  +0.50000  +30°  180°α 
210°  0.50000  30°  180°α 
330°  0.50000  30°  360°+α 
All of the angles in the third colum are between 90° and +90°. This is shown graphically in Figure B5.
Figure B5 ArcSine Angle Range 
A triangle may have an angle which is greater than 90° but if you are solving that angle using the Law of Sines, the computed angle will be less than 90°. Although no single angle in a triangle may exceed 180° the Law of Sines can result in two angles both less than 180°.
For example sin^{1}(0.5) can be 30° or 150°  both meet the sine condition but only one meets the triangle condition. A calculator will always return 30° even though the “true” angle is 150°. How do you know which is correct? It depends on the triangle.
(1) Example
In triangle ABC, a = 12.4', b = 8.7', and B = 36°40'.
Compute the remaining angles and side.
From the Law of Sines
There are two angles whose sine is +0.85112
A = 58°20' and A = (180°00'  58°20') = 121°40', Figure B6.
Figure B6 Multiple Angles with Same Sine 
That means there are two different triangle solutions, Figure B7
Figure B7 Two Possible Angle Values 
Consider point C as the center of an arc of radius 8.7'.
This arc can intersect the remaining side at two points; A_{1} and A_{2}.
Isolating the two triangles and solving their remaining components:
For A = 58°20'
Figure B8 First Solution 
Angle condition
Law of Sines
For A = 121°40'
Figure B9 Second Solution 
Angle condition
Law of Sines
(2) Example 2
Compute the angle B in the following triangle:
Figure B10 Example 2 
The missing angle can be computed two ways:
Law of Sines
Angle condition
As a result there are two "correct" answers fot the missing angle:
sin(108°) = sin(72°) = 0.95105652
Ans since neither exceeds 180°, either can be an angle of a triangle,
but only 108° fits both the Law of Sines and the Angle condition.
Be careful when using the Law of Sines to solve for an unknown angle – there could be two possible answers only one of which will fit the particular triangle. 
b. Cosine; Law of Cosines
The cosine of any angle falls between 1.0 and +1.0. Refer to the cosine curve plot in Figure B11 to see how angles and their cosines relate. This curve repeats itself every 360°; so cos(340°) = cos(20°) = cos(380°)...
The cosine curve is identical to the sine curve except its phase differs by 90°.
Figure B11 
Taking the arccos (a.k.a. inverse cosine, cos^{1}) of a number between +1.0 and 1.0 on a calculator will always return an angle between 0° and 180°; for example:
Table B2  
α  x=sin (α)  sin1(x) 
60°  +0.50000  +60° 
300°  +0.50000  +60° 
120°  0.50000  +120° 
240°  0.50000  +120° 
All of the angles in the third colum are between 0° and +180°. This is shown graphically in Figure B12.
Figure B12 ArcCosine Angle Range 
Using the Law of Cosines will not cause an ambiguous solution as does the Law of Sines since any single angle in a triangle cannot exceed 180°.
To solve a triangle using the Law of Cosines you must have either three sides, or, two sides and an angle.
(1) Example 1
Compute the value of the angle R in the triangle below:
Figure B13 Example 
From the Law of Cosines:
The Law of Cosines returns only one legitimate value when solving triangles.
c. Tangent
Unlike sine and cosine, the tangent of any angle not limited to the range of 1.0 to +1.0. As a matter of fact, the tangent range is ±∞ (infinity). You can see that the tangent function plot is not sinusoidal as are the sine and cosine plots. And unlike the other two it repeats itself every 180°;
Figure B14 
The tangent curve is asymptotic at 90°, 270°, 450°, etc. Asymptotic means the curve gets close to, but never reaches, those values.
so tan(90°) = tan(270°) = ... = infinity
Try evaluating tan(90°) on your calculator; you’ll probably get an error statement of sorts. Then try tan(89.99999°); you should get a pretty big number. Why is that?
Recall that tan(α) = sin(α) / cos(α)
At 90°, cos(90°) = 0 so you get division by 0, hence tan(90°) = ∞.
There is a Law of Tangents, but we don't generally use it to solve triangles since the Laws of Sines or Cosines can be used instead. For extra credit, find and memorize it.