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  4. Consider 8 vertices of a regular octagon and its centre. If T denotes the number of triangles and S denotes the number of straight lines that can be formed with these 9 points then T - S has the value equal to

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Q. Consider 8 vertices of a regular octagon and its centre. If T denotes the number of triangles and S denotes the number of straight lines that can be formed with these 9 points then T - S has the value equal to

Permutations and Combinations

Solution:

$\left({ }^9 C _3-4\right)-{ }^8 C _2=52 \Rightarrow$ (B)