Question
The straight lines I1, I2, I3 are parallel and lie in the same plane. A total number of m points are taken on I1 ; n points on I2 , k points on I3. The maximum number of triangles formed with vertices at these points are -
- m + n + kC3
- m + n + kC3 – mC3 – nC3 – kC3
- mC3 + nC3 + kC3
- None of these
The correct answer is: m + n + kC3 – mC3 – nC3 – kC3
Total number of points = m +n + k. Therefore the total number of triangles formed by these points is m + n + kC3. But out of these m + n + k points, m points lie on I1, n points lie on I2 and k points lie on I3 and by joining three points on the same line we do not obtain a triangle. Hence the total number of triangles is
m + n + kC3 – mC3 – nC3 – kC3.
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strikes the wall with speed 
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, drawn parallel to 
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