Calculator for finding the Area of a circle
   
 Now maybe we know what sines and cosines and stuff like that ARE.
Related Chapters
   
  
 What do we do with them?
 We'll start out with a couple of easy problems and go from there.
 
 Example:

 In the triangle above, 
 we know the length of one of the sides (4) 
 and the measure if two of the angles 
 (remember, the little square means the angle is 90°). 
 Can we figure out the length of the other two sides, 
 and the measure of the third angle?
 
 The angle is easy, and we don't even need trig. 
  We have a 90° angle and a 60° angle.
 Since all of the angles add up to 180°: 
 

180° - 90° - 60° = 30°

 So one piece of the puzzle is in place.

 For the next step we do need trig. 
  The sine of an angle is equal to the opposite side of the triangle
 divided by the hypotenuse (S - O - H), 
 So:

sin 30° = 

 solving this for "h" we get

h =

 "sin 30" is just a number. 
  We can use a calculator or a trig table to find it. 
  sin 30 = 0.5, so we have: 

h =

h =

   8

 Now we have:

 Almost done! The last thing is to find the length of the third side. 
  We could use trig. 
  We could use any one of these:
 

Cos 30°  = 

Adjacent

or

Sin 60° = 

Opposite or

Tan 60° = 

Opposite



8

8 4
 
 Any of these will give you the right answer if you don't make a mistake. 
 We could also use our old friend the Pythagorean theorem.
 

a2 + b2 = h2  

(or call h2, c2 if you want)

 
 So :
42 + b2 =   82
16 + b2 =   64
b2 =   64 - 16
b2 =   48
b  =  
 which you could simplify to:
 
b  =  
b  =  
 Some people might say that this isn't any simpler than the square root of 48, 
  but some instructors like it better if you write it that way. 
 Some will will want you to use a calculator and get a decimal answer (6.9282). 
  Hey, whatever makes them happy.
 
 Let's try another one ...
 
 Example:
 
 Find the angles and the length of the other side.
 First, use Pythagoras to find the length of the side:
 
62+b2 =   152
36+b2 =   225
b2 =   225 - 36
b2 =   189
b =  
b =  
b =  
b =   13.748

 It almost never works out so that all three sides are whole numbers. 
  Now we need to find the angles. 
  It's time for trig. 
  Let's call the pointy little angle on the right A", so:

SinA = 6/15 = 0.4

  If we're using trig tables, find 0.4 in the table and see what angle has 0.4 as it's sine. 
  If you're using a calculator, you MIGHT press these keys:

 I said MIGHT because many calculators have their own special way of doing things. 
  
  Anyway, the angle that has .4 as its sine is about 23.578°.

 Now we use the "three angles inside a triangle add up to 180°" thing
 to find the other angle:
 
90° + 23.578° + the other angle =   180°
the other angle =   180° - 90° - 23.578°
 the other angle =   66.422°

 MORE ABOUT NOTATION:
 During that problem, we used the phrase "the angle that has.4 as its sine".
 That's mouthful !
 Math types just couldn't leave that one alone.
 They had to come up with some word or symbols to mean that. So they did.
 Twice!
 First they came up with the word "arc"
 which they tacked onto the front of sine, tangent and the others 
 to get arcsine, arctan and so on.
 
 So arcsin 0.4 means: "the angle that has 0.4 as its sine"
 Well after a while, they decided that they didn't like that notation any more,
 so they changed it.
 
 Now they use: sin-1 0.4
 It still means: "the angle that has 0.4 as its sine"
 But wait! That looks like an exponent! And it's where the exponent should go!
 All true.
 Personally, 
 I didn't see anything wrong with the old "arctan" type notation,
 but then, nobody asked me.
 

copyright 2008 Bruce Kirkpatrick