Permutations!

There are 'n' points situated on a plane such that no two of the lines joining them are parallel and no three are concurrent except at given points.Find the no. of points of intersection (other than the given points) of the lines btained by joining them.

6 Answers

62
Lokesh Verma ·

***** Posted by me is incomplete! **********

each two lines will meet once..

no of lines will be nC2 = k

we need to chose 2 points out of these..

so the answer is kC2

1
ith_power ·

i dont think above ans is correct. check the pic.

1
ith_power ·

for n=4, ans=3.(A,L,D)

62
Lokesh Verma ·

wait wait :)

yes u are right i need to substract the points that are formed on the original points :)

thanks for pointing out dear

i will try to find that and give the right answer in a moment :)

1
ith_power ·

for n=4, ans=3.(A,L,D)

62
Lokesh Verma ·

each two lines will meet once..

no of lines will be nC2 = k
we need to chose 2 points out of these..
so the answer is kC2

But among these there will be many lines that meet at the original points..

There are n-1 lines thru each point.
they meet in n-1C2 ways

so the answer is
kC2 - n.n-1C2 ways

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