Out of 18 points in a plane, no three are in the same straight line ex

1.	Out of 18 points in a plane, no three are in the same straight line except five points whichare collinear. Find the(1) Number of straight lines formed from the pairs of these points(i) Number of triangles formed with vertices as these points(ii) Number of triangles formed with two vertices from these five collinear points
Answer» First find all possible lines then subtract lines which lie on those five points... To form a line..... There must be at joining of two points... 18c2 - 5c2 then add one line which passes through five collinear points... Answer is 143+1=144

Out of 18 points in a plane, no three are in the same straight line except five points whichare collinear. Find the(1) Number of straight lines formed from the pairs of these points(i) Number of triangles formed with vertices as these points(ii) Number of triangles formed with two vertices from these five collinear points

Answer»

First find all possible lines then subtract lines which lie on those five points...

To form a line..... There must be at joining of two points...

18c2 - 5c2

then add one line which passes through five collinear points...

Answer is 143+1=144