Waste less time on Facebook — follow Brilliant.
×

Is there a pure geometric solution to this problem?

ABCD is a square. And there is a point E such that angle EAB = 15 and angle EBA = 15 degrees. Show that EDC is an equilateral triangle. Now there is a proof by contradiction solution to this problem. I was wondering if there is a pure geometric solution too(no trigonometry)?

Note by Shiv Gaur
3 years, 3 months ago

No vote yet
1 vote

  Easy Math Editor

MarkdownAppears as
*italics* or _italics_ italics
**bold** or __bold__ bold

- bulleted
- list

  • bulleted
  • list

1. numbered
2. list

  1. numbered
  2. list
Note: you must add a full line of space before and after lists for them to show up correctly
paragraph 1

paragraph 2

paragraph 1

paragraph 2

[example link](https://brilliant.org)example link
> This is a quote
This is a quote
    # I indented these lines
    # 4 spaces, and now they show
    # up as a code block.

    print "hello world"
# I indented these lines
# 4 spaces, and now they show
# up as a code block.

print "hello world"
MathAppears as
Remember to wrap math in \( ... \) or \[ ... \] to ensure proper formatting.
2 \times 3 \( 2 \times 3 \)
2^{34} \( 2^{34} \)
a_{i-1} \( a_{i-1} \)
\frac{2}{3} \( \frac{2}{3} \)
\sqrt{2} \( \sqrt{2} \)
\sum_{i=1}^3 \( \sum_{i=1}^3 \)
\sin \theta \( \sin \theta \)
\boxed{123} \( \boxed{123} \)

Comments

Sort by:

Top Newest

Construction of a similar 15deg triangle on BC. Ie. BFC. Then BFE is equilateral. Then by congruency you can do it. Instead of proving CD=CE, prove that CB=CE. Sorry I'm slow with formatting so can't post entire solution. Hope you understand it.

Lavisha Parab - 3 years, 3 months ago

Log in to reply

Proof by contradiction I meant: let angle EDC =x, angle DEC=(180-2x), angle EDA=(90-x), angle DEA=(x+15). Now suppose that x >60. Then 3x>180 therefore x>(180-2x). Therefore angle DCE>angle DEC. Therefore DE>DC. Also, x+15>75, so in triangle DEA we have angle DEA>angle DAE. Therefore AD>DE. Thus we have AD>DE>DC, an absurdity because AD=DC! Likewise for x<60. So the only option left is to conclude x=60 and this means triangle DEC is equilateral.

Shiv Gaur - 3 years, 3 months ago

Log in to reply

x=arctan[2-tan15]

Archit Boobna - 3 years, 3 months ago

Log in to reply

Here is a geometric solution provided by Dr.Shailesh Shirali to me(along the lines suggested by Lavisha Parab):

Draw a copy of triangle EAB with DA as base. That is, locate point F inside the square such that triangle FDA is congruent to triangle EAB.

Then angle FAD = angle EAB = 15 deg, so angle FAE = 60 deg. Also, FA = EA. Hence triangle FAE is equilateral, and angle AFE = 60 deg, and FA = FD = FE.

Therefore F is equidistant from A, D, E and so is the circumcentre of triangle DAE.

Therefore by the circle theorems, angle AFE = 2 angle ADE.

But angle AFE = 60 deg. Hence angle ADE = 30 deg, and it follows that angle CDE = 60 deg. So triangle CDE is equilateral.

(I am unable to upload or paste the jpg of the above solution)

Shiv Gaur - 3 years, 3 months ago

Log in to reply

You can also do it by area. Though I don't know whether that qualifies for "geometry".

Lavisha Parab - 3 years, 3 months ago

Log in to reply

I see it as a pyramid (3D), just looking at the angles we cant really say it as equilateral. But an isosceles!

Sahil Sayed - 3 years, 3 months ago

Log in to reply

I too searched a purely geometrical solution of it. I ended up using atleast the ratio of sides of a right triangle, which i guess not geometry... But let's see if someone comes out with a nice solution.... :)

Sanjeet Raria - 3 years, 3 months ago

Log in to reply

×

Problem Loading...

Note Loading...

Set Loading...