An important mathematical concept is the idea of
function  the value of one variable in a problem depends
uniquely on the value of another variable in the problem.
Changing the value of one variable produces a change in the
value of the other variable that is exactly the same every time
you change the first variable.
Some problems and relations occur so often in math and science that
we assign special names to the function.
The exponential function relates a variable to the number
of times another variable is used as a multiplier, even if
the number of times is not an integer.
The logarithmic function is the inverse of the exponential;
it gives the number of times that you must multiply a variable
by itself to get a given number.
The trigonometric functions
sine, cosine, and tangent
relate the various sides and angles of a triangle.
We can learn about functions by looking at some examples.
The trigonometric functions are defined in terms of the ratio
of the length of the sides of a right triangle and the
claim is made that this ratio depends only on the the value of
the angle formed by two sides of the triangle and not on
the length of any one side.
On this page we show a simple example which demonstrates this
fact.
We are going to use the cosine of the angle in this example, but a
similar example could be made using the sine or the tangent.
Consider the large yellow triangle on the figure. It contains several
other similar triangles which we will be using. All of the triangles are
right triangles and all have the same angle b at the lower left
corner.
The black triangle is the smallest triangle and has an hypotenuse "h" of length "y"
and an adjacent side "a" equal to "x". The length of "x" is arbitrary, but the
value of "y" is not arbitrary, because the
ratio of x to y, (x/y), is defined to be the cosine of "b" and this value
is fixed by "b".
For the black triangle, the cosine of "b" is equal to x / y.
Now let us consider the red triangle.
It was formed by doubling the the size of sides of the
black triangle. For the red triangle, the length of the hypotenuse "h" is 2y,
twice the hypotenuse of the black triangle. Similarly, the adjacent side "a"
is equal to 2x
relative to the black triangle. Both the red
triangle and the black triangle have the same angle "b", in blue. For the red
triangle, the cosine of "b" is equal to 2x / 2y. Dividing out the "2's" gives
x / y, the same value as the black triangle.
Finally, let us consider the blue triangle.
It was formed by tripling the the size of sides of the
black triangle. For the blue triangle, the length of the hypotenuse "h" is 3y,
and the adjacent side "a" is equal to 3x.
For the blue
triangle, the cosine of "b" is equal to 3x / 3y. Dividing out the "3's" gives
x / y, the same value as the black triangle and the red triangle.
If we quadruple or increase the size of the sides by any multiplier,
the ratio of the adjacent side to the hypotenuse stays the same x / y.
The value of cosine "b" just depends on the value of "b" and not the length
of the sides that form "b". Scientists call this type of dependence a function. The ratio of the adjacent side to the hypotenuse is a function of the
angle "b".
Since the cosine, and the sine and tangent, are all functions of the angle "b", we can
determine (measure) the ratios once and produce tables of the values of the
sine, cosine, and tangent for various values of "b". Later, if we know the
value of an angle in a right triangle, the tables will tell us the ratio
of the sides of the triangle.
If we know the length of any one side, we can solve for the length of the other sides.
Or if we know the ratio of any two sides of a right triangle, we can
find the value of the angle between the sides.
We can use the tables to solve problems.
Some examples of problems involving triangles and angles include the
descent
of a glider, the
torque
on a hinge, the operation of the Wrights'
lift,
and
drag balances,
and determining the
lift to drag
ratio for an aircraft.
Here are tables of the sine, cosine, and tangent which you can use to solve
problems.
Activities:
Navigation..
 ReLiving the Wright Way
 Beginner's Guide to Aeronautics
 NASA Home Page
 http://www.nasa.gov
