Special Right Triangle 30º- 60º- 90º Topic Index | Algebra2/Trig Index | Regents Exam Prep Center

The
30º- 60º- 90º triangle is one of two special right triangles we will be investigating.  The "special" nature of these triangles is their ability to yield exact answers instead of decimal approximations when dealing with trigonometric functions.

 If you draw an altitude in an equilateral triangle, you will form two congruent 30º- 60º- 90º triangles.  Starting with the sides of the equilateral triangle to be 2, the Pythagorean Theorem will allow us to establish pattern relationships between the sides of a 30º- 60º- 90º triangle.  These relationships will be stated here as "short cut formulas" that will allow us to quickly arrive at answers regarding side lengths without applying trigonometric functions, or other means. There are three pattern relationships that we can establish that apply ONLY to a 30º-60º-90º triangle.

Note:  the hypotenuse need not be a length of 2 for these patterns to apply.
The patterns will apply with any length hypotenuse.

30º-60º-90º Triangle Pattern Formulas
(you do not need to memorize these formulas as such, but you do need to memorize the relationships)

 Labeling: H = hypotenuse LL = long leg (across from 60º) SL = short leg (across from 30º)

Short Cut Pattern Formulas:
(These formulas give answers directly.
We don't need to work out the trig,
as we already know the pattern relationships)

 short leg: You must remember that these formula patterns can be used ONLY in a 30º-60º-90º triangle. long leg: combining the first two:

Using the patterns to find the lengths of sides:

 EASY: Find x and y. x is the short leg   Answer y is the long leg  Answer

 HARDER: Find x and y. 6 is the short leg and x is the hypotenuse (start with what you have given)   Answer y is the long leg  Answer

 HARDER: (requires more algebraic manipulation) Find x and y. 8 is the long leg and  x is the hypotenuse (start with what you have given)   Answer y is the short leg  Answer

Using the newly found patterns in trig problems:

 1.  Find the exact value of            tan 30º + cos 60º. Solution:

 2.  Find the exact value of                  (sec 30º)2. Solution:

 What if I forget the formula patterns? What should I do?

There is always more than one way to tackle a problem.  If you forget these formula patterns, you could always use the trigonometry formulas to find one of the missing sides and then use the Pythagorean Theorem to find the last side OR you could use the trigonometry formulas to find both missing sides.

Unfortunately, the Pythagorean Theorem by itself,
will not help you find both of the missing sides.  Remember that you need to know TWO sides of a triangle in order to engage the Pythagorean Theorem.

 Topic Index | Algebra2/Trig Index | Regents Exam Prep Center Created by Donna Roberts