Thursday, March 8, 2012

Inverse Trigonometric Functions - Arcsine, Arccosine, Arctangent

Inverse trig functions are a core concept in trigonometry.  Specifically, they are named arcsine, arccosine, and arctangent.  We have already discussed how to find things such as the sine of 25 degrees, or cosine of 71 degrees.  However, we need to introduce a new math concept to figure out such problems as "what angle gives a sine value of 0.2?"  This is where inverse trig functions come in. Think of their relationship to the standard basic trig functions as being comparable to that between multiplication and division.  You do one operation, and then the opposite to go the other way and undo the first operation.

You may be taught these in school slightly differently.  Instead of using the names arcsine, arccosine, and arctangent (arcfunctions!), it is also very common to see these represented as the basic trig notation with a -1 exponent, like these:

arcsine = sin-1
arccosine = cos-1
arctangent = tan-1

These are likely the symbols that you will see on your calculator.  Typically, they are the shifted function of the regular sin, cos, and tan buttons.  It is important to note the distinction between the basic trigonometric functions and these new inverse trig functions.  When using the basic trig functions, the value you are obtaining is the ratio of the two relevant sides of the triangle for the given angle.  When using the inverse trig functions, what you are solving for is the actual angle that produces the given ratio of sides.  So, make sure you push the right button on the calculator!

The most basic way of finding an inverse function, in general, is to take your given function and switch the x and y, and then rearrange.

f(x) = x2 + 5
y = x2 + 5.... now switch x and y
x = y2 +5.... and rearrange
y2 = x - 5
y = sqrt(x-5)

And there you have determined the inverse function.  This is the same strategy that is being applied when we are talking about inverse trig functions.  However, having the inverse trig buttons on our calculators really take all of this extensive and possibly difficult rearranging and calculating out of the picture.

Here is a basic example of one of these inverse trigonometric functions.  Hopefully you will see that they are extremely easy to work with.

Find the angle for the given trig ratio:
sin(θ) = 1 /√2
θ = sin-1(1 /√2)
θ = 45°

Hopefully this introduction to the inverse trig functions has been useful for you.  There is a lot more information about this math concept that is probably quite beyond the scope of what is necessary to actually solve most of the basic trigonometry questions you will find in your maths homework.  I have found several good resources, if you would like to learn more about inverse trigonometric functions.  They can be found on the math website, or on The Math Page (among many other great math resources out there).  Please leave a comment below if you know of any other great sites that you would like to share with other readers. Also, once again, please hit the +1 if you've enjoyed this post!

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