Math Is Fun Forum

philaretus

Member

Registered: 2016-01-13

Posts: 6

intro to geometry

1)  ABCDE is a regular Pentagon. find the angles of  triangle ADC.

2).  A regular polygon has angles of size 150 degrees each. How many sides has the polygon?

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: intro to geometry

Hi;

2) Each angle of a regular polygon is given by the formula

where n is the number of sides. So just set that to 150

Expand out.

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,621

Re: intro to geometry

Hi philaretus

Using that formula of bobbym's with n = 5, the interior angle of a regular pentagon is

So angle AED = 108, and the triangle is isosceles.  Use the angle sum of a triangle to work out EAD.

BAC will be the same (by symmetry) so you can work out DAC.  That triangle is also isosceles so ADC = ACD and can be worked out from there.

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

Monox D. I-Fly

Member

From: Indonesia

Registered: 2015-12-02

Posts: 2,000

Re: intro to geometry

Unfortunately, in my country people tend to think that all regular polygons consist of equilateral triangles (they think that regular pentagon consists of 5 equilateral triangles, etc). This is sad. Even the equilateral triangle doesn't consist of three smaller equilateral triangles. They don't know that it only applies to regular hexagon.

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Actually I never watch Star Wars and not interested in it anyway, but I choose a Yoda card as my avatar in honor of our great friend bobbym who has passed away.
May his adventurous soul rest in peace at heaven.