Please turn on javascript in your browser to play chess.
Posers and Puzzles
RHP Arms
Joined 09 Jun '07 Moves 48793 Here is an easy geometry puzzle for those that cannot do Ranjan's stinkers!
What is the area of an isoceles triangle with a base of 12 and an apex of 120 degrees.
Elegant and short proof required. 🙂
Out of my mind
Joined 25 Oct '02 Moves 20443 Originally posted by wolfgang59
Here is an easy geometry puzzle for those that cannot do Ranjan's stinkers!
What is the area of an isoceles triangle with a base of 12 and an apex of 120 degrees.
Elegant and short proof required. 🙂 Divide the triangle in two equal parts.
Each of the halves is a 30-60-90 triangle with middle side 6.
This means that the short side (wich is the height of the original triangle) of the halves is 6/sqrt(3).
So the area of the original triangle is 1/2 * 12 * 6/sqrt(3) = 12 * sqrt(3)
Elegant enough?
RHP Arms
Joined 09 Jun '07 Moves 48793 Correct but you have assumed the reader knows ratios of the sides of a 30, 60, 90 triangle.
I thought a neater idea was to spot that the described triangle is a third of an equilateral triangle of side 12.
So using your shortcut we have area of equilateral triangle is
0.5 * 12 * 6sqrt3 = 36sqrt3
Therefore isoceles is 12sqrt3
I said it was easy!!
Out of my mind
Joined 25 Oct '02 Moves 20443 Originally posted by wolfgang59
Correct but you have assumed the reader knows ratios of the sides of a 30, 60, 90 triangle.
I thought a neater idea was to spot that the described triangle is a third of an equilateral triangle of side 12.
So using your shortcut we have area of equilateral triangle is
0.5 * 12 * 6sqrt3 = 36sqrt3
Therefore isoceles is 12sqrt3
I said it was easy!! Ah, that is also a nice solution 🙂
H. T. & E. hte
Joined 21 May '04 Moves 3510 Originally posted by wolfgang59
Correct but you have assumed the reader knows ratios of the sides of a 30, 60, 90 triangle.
I thought a neater idea was to spot that the described triangle is a third of an equilateral triangle of side 12.
So using your shortcut we have area of equilateral triangle is
0.5 * 12 * 6sqrt3 = 36sqrt3
Therefore isoceles is 12sqrt3
I said it was easy!! Really elegant. Also...it can be viewed as being equal to the area of an equilateral triangle of side 12/(sqrt(3)).
The two halves of this equilateral triangle having been juxtaposed to form the given isosceles triangle with vertex angle 120 deg...
Cookies help us deliver our Services. By using our Services or clicking I agree, you agree to our use of cookies. Learn More . I Agree