A rightangled triangle is a
triangle in which one of the angles is a rightangle. The hypotenuse of a right
angled triangle is the longest side, which is the one opposite the right angle.
The adjacent side is the side which is between the angle in question and the
right angle. The opposite side is opposite the angle in question.
In any right angled
triangle, for any angle:
The sine of the angle = the length of the
opposite side
the length of the hypotenuse
The
cosine of the angle = the length of the adjacent side the length of the
hypotenuse
The tangent of the angle = the length of the opposite
side
the length of the adjacent side
So in shorthand notation:
sin = o/h cos = a/h tan = o/a Often remembered by: soh cah toa
Example
Find the length of side x in
the diagram below:
The angle is 60 degrees. We
are given the hypotenuse and need to find the adjacent side. This formula which
connects these three is: cos(angle) = adjacent / hypotenuse therefore,
cos60 = x / 13 therefore, x = 13 × cos60 = 6.5 therefore the length of
side x is 6.5cm.
The Graphs of Sin, Cos and Tan
The following graphs show the value of sinų, cosų and tanų against ų (ų
represents an angle). From the sin graph we can see that sinų = 0 when ų = 0
degrees, 180 degrees and 360 degrees.
Note that the graph of tan has asymptotes (lines
which the graph gets close to, but never crosses). These are the red lines (they
aren't actually part of the graph).
Also notice that the graphs of sin, cos and tan
are periodic. This means that they repeat themselves. Therefore sin(ų) = sin(360
+ ų), for example.
Notice also the symmetry of the graphs. For
example, cos is symmetrical in the yaxis, which means that cosų = cos(ų). So,
for example, cos(30) = cos(30). Also, sin x = sin (180  x) because of the
symmetry of sin in the line ų = 90.
Copyright © Matthew Pinkney 2003
