1. Tardigrade
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  3. Mathematics
  4. There are 15 points in a plane, no three of which are in a straight line, except 6, all of which are in a st. line. The number of st. lines which can be drawn by joining them is

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Q. There are $15$ points in a plane, no three of which are in a straight line, except $6$, all of which are in a st. line. The number of st. lines which can be drawn by joining them is

Permutations and Combinations

Solution:

Reqd. no. of st. lines $= ^{15}C_{2}-\,{}^{6}C_{2}+1$
[$\because$ join of two pts. from a st. line and $6$ pts. which are collinear given only st. line in place of $^{6}c_{2}$]