There are 15 points in a plane of which 8 of them are on a straight line..then how many
1) straight lines can be formed
2)triangles can be formed
3)difference between them?

• Sorry did a Minor Mistake
  Corrected Solution:
  1) 15C2 - (8C2 - 1) = 15C2 - 8C2 + 1 = 78
  2) 15C3 - 8C3 = 399
  3) 399 - 78 = 321
  
• 10 years agoHelpfull: Yes(42) No(1)
• 1. 15C2 - 8C2 + 1 = 78
  2. For a triangle we need 3 points
  15C3 - 8C3 = 399
  3. 399 - 78 = 321
• 10 years agoHelpfull: Yes(7) No(0)
• 1) 15C2 - (8C2 - 1) = 15C2 - 8C2 + 1 = 78
  2) 15C2 - 8C2 = 77
  3) 78 - 77 = 1
• 10 years agoHelpfull: Yes(2) No(8)
• 2) for triangle need three points take 8 points in straight and 7 points randomly, we can't make triangle two choose three points in 8 points because they are in a line(straight).
  so required equation will be
  2) 8c2*7c1 + 7c2*8c1 + 7c3 =399
  there is also possibility of triangle to choose three points among 7 points they are random so triangle will be formed.
• 10 years agoHelpfull: Yes(2) No(0)
• 1) 15C2-8C2+1 = 78
  2) 15C3-8C3 = 399
  3) 399-78 = 321
• 10 years agoHelpfull: Yes(1) No(0)
• 1)15c2+8c2-1
  2)15c3+8c3
  3)1-2
• 10 years agoHelpfull: Yes(0) No(0)
• why we are taking (-1) in first step..???any bady tell me
• 9 years agoHelpfull: Yes(0) No(0)
• Here we can form a straight line using two points...whether they lie on same line.so total no of straight line would be 15C2= 85.
  No of triangles would be 15C3-8C3=399
  Finally difference equals to 399-85=314
• 9 years agoHelpfull: Yes(0) No(0)