# The 27-63-90 Triangle I’m not as pleased about finding this special right triangle as I was with the previous ones. For one thing, I didn’t use a regular polygon to derive it, but instead used the 36-54-90 triangle I had previously found, and applied a half-angle trigonometric identity to find the tangent of 27 degrees, as the tangent of half of 54 degrees. According to this identity, tan(27⁰) = (1 – cos(54⁰))/sin(54⁰). Once this gave me the leg lengths, I simply used the Pythagorean Theorem to determine the length of the hypotenuse. Finally, I checked all of this using decimal approximations.

The triple-nested radical in the expression for the hypotenuse is no cause for celebration, either. If anyone knows of a way to put this in simpler exact terms, please let me know.

## 3 thoughts on “The 27-63-90 Triangle”

1. Anonymous on said:

how did you come up figure out that would work though?

Like

2. It’s derived from the 36-54-90 triangle, which is itself derived from the regular pentagon, and uses the fact that such pentagons’ diagonals and sides are in the golden ratio.

Like

3. Anonymous on said:

The hypotenuse is equal to: 2*sqrt[5+sqrt(5)]-sqrt(2)*(sqrt(5)-1).

Like