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Out of 18 points in a plane, no three ar...

Out of 18 points in a plane, no three are in the same line except 5 points which are collinear How many straight lines can be formed by joining the given points?

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Number of stràight lines `= overset(n) C_2-( ^circ C_2-1)`
`= ^18 C_2-( ^5 C_2-1)=153-9=144(because, n=18, p=5)`
Number of triangles `= overset(n) C_3- ^p C_3= ^18 C_3- ^5 C_3=816-10=806`
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