94 Hsc Probability question (1 Viewer)

[GaganDeep]

CAn some one explain the answer for 94 Hsc q7 b)
9 points lie on a place and 5 on a line l
No other set of 3 of these points is colllinear
1. How many sets of 3 points can be chosen from the 5 points lying on l?
2. how many different triangles can be formed using 9 points are vertices?
i can do part 1. 5C3

 

[pLuvia]

2. 9C3-5C3
Since those collinear points can't make a triangle

 

[rocky1989]

if you think about it....take any 3 of those 5 points that are all on the same line and the shape they make isn't quite a triangle, so you have to deduct the number of combinations that have three points that won't even make triangles