MHB Number of quadrilatera in polygon

[juantheron]

Number of Quadrilateral that can be made using the vertex of a polygon of 10 sides as

there vertices and having Exactly $2$ sides common with the polygon

My solution:

first we will take $2$ sides common is $=10$ ways now if we take two sides as $A_{1}A_{2}$ and $A_{1}A_{3}$ then we

will not take vertex $A_{4}$ and $A_{10}$ So we will choose $1$ vertes from $5$ vertices which can be done by

$\displaystyle \binom{5}{1}$ ways So Total No. of ways in which $2$ sides common is $=10 \times \displaystyle\binom{5}{1} =50$ but answer given

is $=75$.

But I did not understand it. Would anyone like to explain me where i am wrong

Thanks

 

[caffeinemachine]

Re: number of quadrilatera in polygon

Number of Quadrilateral that can be made using the vertex of a polygon of 10 sides as

there vertices and having Exactly $2$ sides common with the polygon

My solution:

first we will take $2$ sides common is $=10$ ways now if we take two sides as $A_{1}A_{2}$ and $A_{1}A_{3}$ then we

will not take vertex $A_{4}$ and $A_{10}$ So we will choose $1$ vertes from $5$ vertices which can be done by

$\displaystyle \binom{5}{1}$ ways So Total No. of ways in which $2$ sides common is $=10 \times \displaystyle\binom{5}{1} =50$ but answer given

is $=75$.

But I did not understand it. Would anyone like to explain me where i am wrong

Thanks

You can also have a quadrilateral $A_1A_2A_4A_5$. It too has exactly $2$ of its sides common with the polygon. There, of course, many more such quadrilaterals.

 

[soroban]

Hello, jacks!

I think I've got it . . .

Number of quadrilateral that can be made using the vertices
of a polygon of 10 sides as their vertices and having
exactly 2 sides common with the polygon.

Answer: 75

              A   B

          J           C        I               D

 
          H           E

              G   F

Case 1: the two sides are not adjacent.

The first side can be any of the 10 adjacent vertex pairs:
. . AB, BC, CD, \text{ . . . } JA

Suppose the first side is AB.
Then the second side has 5 choices:
. . DE, EF, FG, GH, HI

It seems there are 10\times 5 \,=\,50 such quadrilaterals.
But this list includes \{AB,FG\} and \{FG,AB\}

Hence, there are: \tfrac{50}{2}\,=\,25 such quadrilaterals.Case 2: The two sides are adjacent.
There are 10 triples of vertices:
. . ABC,BCD,CDE,\text{ . . . }JAB

Suppose the triple is ABC.
Then the fourth vertex can be: \{E,F,G,H,I\}

Hence, there are: 10\times5\,=\,50 such quadrilaterals.Therefore, there are: 25 + 50 \,=\,75 quadrilaterals.