Trigonometry

Trigonometry takes on two distinct forms:

• right triangle trig
• oscillating function trig

 

Each one plays a huge role in basic physics, calculus and engineering

            right triangle trig:  force diagrams, geometry, vector analysis

            oscillating functions:  calculus,  mass-spring systems, circuits, sound,
and any vibrations

 

Contents:

            1. Right Triangle Trig
1.1 Pythagorean Theorem
                        1.2 SOH CAH TOA  and similar triangles
                        1.3 Trig values for 0°, 30°,45°,60°, and 90°
                        1.4  Degrees and Radians
                        1.5 Trig Identities

            2. Oscillating Functions
                   2.1 basics of sine and cosine
                        2.2 period
                        2.3 frequency
                        2.4 phase shift
                        2.5 standing waves

1. Right Triangle Trigonometry

• The Pythagorean Theorem

 

The Pythagorean Theorem is the cornerstone of trigonometry and analytic geometry. It relates the lengths of the sides of right triangles.  First you need to remember the terminology:

If we denote the lengths of the three sides by a and b for the legs and c for the hypotenuse, then in a flat plane the Pythagorean Theorem states that

                                    c2 = a2 + b2

The most widely known right triangle is perhaps the “3-4-5” triangle shown below

 

 

where we note that  32 + 42 = 9 + 16 = 25 = 52.  Other right triangles with integer sides are  5-12-13 and 20-21-29. The reader should check that the two sets of numbers really do satisfy the Pythagorean Theorem.

Skill: Completing the Triangle

One thing that one should be able to do is as follows: given two sides of a right triangle, be able to fill in the length of the third side.  This is called “completing the triangle”.

Example 1:  Complete the following triangle

Solution:  we must have  x2 + 32 = 72  
                                    or  x2 + 9 = 49
                                    or     x2 = 49 – 9 = 40
                                    so    x =
(most problems result in a square root appearing somewhere…)

Example 2:  Complete the following triangle

Solution:  We need to determine the hypotenuse this time.  We have

                        92 + 132  = x2

            or         81 + 169 = x2
           
or           250       = x2
so     x     =

Problem:  why is it true that  a + b > c   ??

 

1.2 SOH CAH TOA

If one of the angles (not the 90 degree one) are denoted by  θ and we distinguish the legs as to being adjacent to the angle or opposite it then the triangle looks like

 

 

The Pythagorean Theorem in this case states

                                    Hyp2  =  Adj2   + Opp2

The basic trig functions are defined as ratios of the sides of the triangle as the legs relate to θ  (either adjacent to it or opposite it).

 

You need to memorize these!!! They are pronounced:

                          sin(θ)    =  “sine theta”
                    cos(θ)   =  “cosine theta”
                    tan(θ)   =  “tangent theta”

For the right triangle shown below, we would then have

that        sin(θ) = 10/26 = 5/13,
  cos(θ) = 24/26 = 12/13
and         tan(θ) = 10/24 = 5/12.

 

Sometimes you have to complete the triangle first before you can figure out the values of the trig functions as shown by the following example

Example    For the triangle shown below, determine   sin(θ), cos(θ)  and tan(θ).

Solution:   step 1 – complete the triangle

the adjacent side  is found by solving   x2 + 42 = 92   or  x2 = 81 – 16 =65 so
x =

                  step 2 – read off the trig values

so that  

that        sin(θ) =   4/9
  cos(θ) = /9
and         tan(θ) =   4/

Complementary Angles

It is well known that for any triangle, the sum of all three angles is always 180°.

Now for a right triangle, one of those angles is 90°,  so the other  two angles add up to 90°.  They are called complementary because of this.  Thus if you have a right triangle and you know one angle, you really know all three.  If, for example, one angle is 37 degrees than the other must be 63 degrees and the triangle might look like

 

 

Knowing this and SOH CAH TOA can save us some work.  We start with the following triangle and base what follows on it:

Now, remembering SOH CAH TOA, we can write down all of the following:

sin(θ) = Opp/Hyp

cos(θ)  = Adj/ Hyp

as well as (check this)

sin(θc) = Adj/Hyp

cos(θc) = Opp/Hyp

because of where θc   is located on the triangle. Putting this together,

cos(θc) = sin(θ)         and   sin(θc) = cos(θ)

or, equivalently

sin(θ) = cos(90° - θ)

           
or, put verbally: if you know sine of an angle then you know cosine of its complement, and if you know cosine of an angle, you know sine of its complement.

For example,  if we are given that   sin(40°) = .643   then we automatically know that cos(60°) = .643  .

 If we know that   cos(55°) = .82  then we know from this that   sin(35°) = .82. 

Part of geometry and, in particular trigonometry, is getting all the possible mileage you can out of the given information in a problem…. deductive reasoning.

 

1.3  Trig values for 0°, 30°,45°,60°, and 90°

You will frequently need to know the  values of sine, cosine and  tangent for the angles listed above. What follows will try to cut down on the amount of memorization and also explain where the values come from. For starters, we will focus only on sine and cosine and then fall back on the fact that  tan(θ) = sin(θ)/ cos(θ).

Our goal is to fill in the following table based on what we know:

 

θ	sin(θ)	cos(θ)
0°
30°
45°
60°
90°

 

45° is a good place to begin.  The triangle may  look like

if we make the hypotenuse 1.  Note the second 45° because they have to add up to 90°.  This makes the triangle isosceles and hence Opp = Adj . By the Pythagorean Theorem, we then have

            12 = Adj2  + Opp2  =  Adj2 + Adj2  = 2Adj2

so   Adj2 = ½   and therefore  Adj =
Therefore the triangle looks like 

and thus  cos(45°)= sin(45°)= .

The table now looks like

θ	sin(θ)	cos(θ)
0°
30°
45°
60°
90°

 

Let’s set up an equilateral *triangle with sides 1 unit long:

*equilateral means that all three sides are the same length. Isosceles means two equal sides

Now, you point out, this is not a right triangle and we were doing trigonometry, the study of right triangles.  True… but let’s add a line from the left vertex to the midpoint of the vertical side. Now things look like

and we note that the green, horizontal line is perpendicular to the vertical, blue line. We have created two, congruent, right triangles, called “30-60-90” triangles. We have 2 of the three sides of each. If we let x denote the length of the green, horizontal side then one of them looks like

                       

where, from the Pythagorean Theorem, we have  x2 + (1/2)2 = 12   so x =
and the triangle, completed, is

From SOH CAH TOA, we get sin(30°)= = cos(60°)   and  cos(30°) =  = sin(60°)
The table now looks like

θ	sin(θ)	cos(θ)
0°
30°
45°
60°
90°

and we note that because of the property of complementary angles discussed earlier, every time we determine 2 trig values, we effectively get 2 more free of charge. Thus if we can figure out sin and cosine of 0°  we are really done.

For 0°  we can start with a very, very slim triangle with hypotenuse of 1.  This might appear as

                       

Remembering that sine is opposite divided by 1  and cosine is adjacent divided by 1, we then imagine θ  getting smaller and smaller and becoming 0. The adjacent and the hypotenuse both become the same thing so cos(0°) = 1.

By similar thinking, the opposite side becomes 0 as the angle collapses, so  sin(0°) = 0.

The table now looks like

θ	sin(θ)	cos(θ)
0°	0	1
30°
45°
60°
90°

For 90 degrees the role of sine and cosine are reversed so we can fill in the last row as

θ	sin(θ)	cos(θ)
0°	0	1
30°
45°
60°
90°	1	0

The property of complementary angles has saved us half the work. It can save you half of the memorization by the following pattern of duplication of values (indicated by corresponding colors):

θ	sin(θ)	cos(θ)
0°	0	1
30°
45°
60°
90°	1	0

where each pairing occurs for the simple reason that  sin(θ) = cos(90° - θ)

Completing Right Triangles Using Trig Functions

This is a skill where, given only the hypotenuse and an angle in a right triangle, one can fill in all three sides of the triangle.

For example, consider the following problem:

Problem:  determine all three sides of the triangle shown:

Solution:

Let’s find the adjacent (horizontal) side first.  Call it’s value x.  Then by definition, the ratio of  adjacent to hypotenuse is the cosine of 30 degrees, or

                                    cos(30°) = x/20
= 

so   x = 20 = 10√3
To next find the opposite (vertical) side, we have two ways to do it: use the Pythagorean Theorem or use trig functions as we just did. We’ll go with the latter to illustrate it as you already know how to use the Pythagorean Theorem

Call this unknown side y.  Then by definition

                                    sin(30°) = y/20
=  1/2 

                                    so  y = 20(1/2) = 10  

 

                                   
The triangle, now completed, looks like 

Another approach:   we developed the trig values for 30 degrees using a triangle with a hypotenuse of 1 unit and adjacent angle of 30.  What we needed in this problem was a triangle 20 times as big and similar (in the mathematical sense) to the original so we just multiply or scale  all three sides by 20 and we are done

           

Comment: to do these kinds of problems , you need to know SOH CAH TOA cold as well as the values from the trig table. There is no way around this.

 

Problem:  complete the following triangle

Solution:

let’s get the hypotenuse first.  Call it H.  By definition

                                               sin(30) = 8/H

We know sin(30) = ½    so       ½ = 8/H   or   H = 8/(1/2) = 16 

To get the horizontal side, which we call A, we know that

                                    cos(30) = A/H = A/16

and, from the table, that cos(30) =    so

                                    A/16 =
and     A = 16 = 8

The completed triangle now looks like

Comment: we can check our work by seeing if our three sides satisfy the Pythagorean Theorem. If not, we made a mistake.  In this problem,

                       

so we are ok.

Problem:  complete the following triangle

Solution:

let’s call the vertical side y and the hypotenuse h.  Then we are looking for values to complete

 

Let’s find h first.   By definition,  cos(15°) = 12/h  so  h = 12/cos(15).  To finish, we need the cosine of 15 degrees which requires a calculator. This tells us cos(15°) is about .966 so

                                               h = 12/.966  =  12.42

We now have two out of three sides. We can get y either with the Pythagorean Theorem or Trig functions. We will show both

Pythagorean Theorem:       
h2  =  y2  + 122

                                               (12.423)2 = y2 + 144

• =  y2  + 144

 

   10.34 =  y2

     3.22 = y

Trig functions:

                                    sin(15°) = y/h = y/12.42

                                    but  sin(15°) = .259

so                                y = 12.42 * .259  = 3.21

The “completed” triangle now looks like

           

           

Since decimals were used, roundoff is always going on, so the answers are only 2 decimal place approximations. Your calculator probably allows as much as 8 places.

• Degrees and Radians

 

Depending on the books you use or courses you take, angles may be specified in either degrees or radians. You need to be competent in both. This is easy.

First we assume you are familiar with degrees and know there are 360 degrees in a complete circle.

Secondly we  point out that there are also  2π radians in a circle, or about 6.28 radians because  π is approximately 3.14

Thus we have the critical conversion formula

                        360°  =  2π radians
or
                        180°  =  π radians

This last formula is really all you need to remember!!  Just multiply or divide accordingly and you can make any conversion.

For example:

                   60°  =  π/3 radians    (just divide by 3)

                   90°  =  π/2 radians

                   150°  =  (5/6) π radians

with this kind of simple algebra, we can rebuild our trig table from earlier:

 

θ (deg)	θ (rad)	sin(θ)	cos(θ)
0°	0	0	1
30°	π/6
45°	π/4
60°	π/3
90°	π/2	1	0

and comment that you really should memorize both ways, degrees and radians.

Problem :  What about the other direction?

What does 2 radians convert to in degrees? We have  from above that

180°  =  π radians

Dividing both sides by π we have what 1 radian is

                        180°/ π  =  1 radian

Multiplying both sides by 2 gives us

                        360/ π° = 2 radians

Now if you get out your calculator and divide 360 by 3.14 you find we have

                        114.59° = 2 radians
to finish the problem. Note we carried the units all the way along for clarity.

 

Now while we are at it, we can divide by 2 and find the useful fact that

                57.29° = 1 radian 

 

 

 

• Trig Identities

The Pythagorean Theorem says that, for our original right triangle,

                        Hyp2  = Adj2  + Opp2

 

Dividing both sides by Hyp2  we then easily have

                

and if we remember SOH CAH TOA, this is the same as

1 = sin2(θ)  + cos2(θ)     (trig identity #1)

this is a very useful identity and should be memorized. It is really nothing more than the Pythagorean Theorem applied to sine and cosine.

From SOH CAH TOA we have that

            tan(θ =  Opp/Adj

but      sin(θ) = Opp/Hyp   and cos(θ)= Adj/Hyp so

        

so           tan(θ)  = sin(θ)/cos(θ))        (trig identity #2 )

If we divide both sides of trig identity #1  by cos2(θ)  then we have

or, since  1/cos  is defined as secant,

 

sec2(θ)   = tan2(θ)  + 1    (trig identity #3)

This identity appears often in calculus courses.

Other identities:

            sin(2θ) = 2 sin(θ) sin(θ)
           
            sin2(θ) =

            cos2(θ) =

which are used in Calculus II and Differential Equations, among other places.

2. Oscillating Functions

The other use of trig functions is for oscillations.  These occur in vibrations in physics and mechanical engineering as well as electrical circuits.

2.1 Basics of Sine and Cosine

For starters we begin with a circle of radius one and a radius from the origin to a point (x,y) on it.

Now if we drop a perpendicular to the x axis then we have a familiar looking right triangle:

and if  θ  is the angle between the x-axis and the radius then the horizontal and vertical sides, respectively, have lengths of  cos(θ)  and sin(θ).  Remembering how Cartesian coordinates are plotted, this means that

                                    x = cos(θ)    and  y=sin(θ)

Now at this point, this is nothing more than we did earlier, just superimposed on the x-y coordinate system. We even have a table of the coordinates of x and y for 5 different values of θ.
What becomes new is when θ exceeds 90° and we move in to the other quadrants instead of remaining in the first quadrant.

We need to take a second and review the signs (+, -)  of points in other quadrants:

If, for θ > 90°, we define  x as cos(θ)   and y as sin(θ)  then  the functions take on both positive and negative values.

The graphs of cosine and sine, plotted versus the angle  θ, look like:

where cosine is in green and sine is in red. The reader should compare the values on the graphs above, for the first 90 degrees, with those obtained earlier and put into the table

θ	sin(θ)	cos(θ)
0°	0	1
30°
45°
60°
90°	1	0

The only trouble with this is that the values in the table are in fraction form. If they are converted to decimals then it might be easier to relate to the graph. This is done below:

 

θ	sin(θ)	cos(θ)
0°	0.00	1.00
30°	.500	.866
45°	.707	.707
60°	.866	.500
90°	1.00	0.00