Tuesday, June 10, 2008

Special Angles in Trigonometry - Part 2 (Similar Triangles)

This post will hopefully clarify how to work with special triangles when the sides are not the standard lengths (as I described in the original post for Special Angles).

Maybe this would be a good time to describe SIMILAR triangles. Similar triangles are triangles that have different side lengths, but have the same angles. Don't let the fancy name fool you... just think of them as smaller or larger versions of the same triangle. Examine this picture:

If you were to cut out all the triangles, and shrink or enlarge them, you would see that they all would fit on top of each other. This is possible because each triangle has the same angles, despite having different side lengths. When triangles are like this, they are said to be SIMILAR.

Now, to apply this to my previous post explaining the special angles in trigonometry. I explained how to derive the trig functions using the simplest triangles. However, in all likelihood, you will find triangles of different sizes, rather than these same simple triangles. You need only remember the rules of SOHCAHTOA to be able to evaulate the trig functions.

For example:

Take a right angle triangle with an angle of 30 degrees, and you know that the short side is a length of, say, 5. (Try to sketch this out... being able to draw a triangle from a description is important to learn as well!) There are a few strategies you could use here to solve the other 2 sides and last angle.

1) You know that all the angles in a triangle sum up to 180 degrees. So, 180-90-30 = 60!

2) You can apply the rules of SOHCAHTOA to determine either one of both of the unknown sides. In this case, you have the 30 degree angle known, and the short side is opposite this angle. So, you can use the Sine function to determine the length of the hypotenuse (try it! how do you solve Sin(30). Once you have the hypotenuse figured out you can then turn around and use it to to solve the Cosine function... OR you could just use the Theorem of Pythagoras, since it is a right-angle triangle.

3) Remember that you could also use either the Sine Law or Cosine Law in there as well (especially on triangles that are NOT right-angle triangles)... Sine Law works whenever you know an angle and it's opposite side, and then either of an angle or a side to complete the identity. Or the Cosine Law will work when you know 2 sides and the angle between those 2 sides.

Hopefully this quick post addresses some concerns that some of you may have had. Sorry about any confusion or lack of clarity in the first post. As always, please continue to drop me a line if you have any questions, concerns, or topics you would like me to cover!

**EDIT** - My apologies... as pointed out in the comment section, triangles as I have discussed in this post are in fact called SIMILAR, and not CONGRUENT (I have edited the post to reflect this). Similar triangles are triangles with the same angles but can have differing side lengths. Congruent triangles, on the other hand, have the same angles AND sides. This means they look either the exact same, or are a mirror image of the original. Sorry for the confusion, and thanks to the astute reader who picked out my error. :)


  1. If two triangles are of different sizes but of the same shape, are they called congruent or similar?

  2. Triangles of same shape but different size are SIMILAR. Thanks for pointing out my error. I have corrected the posting. :)

  3. i dont have a clue ..

    So if two triangles have the same angles but have different side lenghts they are Similar



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