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Question
Find the number of triangles formed by joining 12 points if four points are collinear.
Solution
There are 12 points 1n a plane of which four points are collinear.
If no three points are collinear, we will get 1003 triangles. Since four points are collinear, the number of triangles will reduce by 403.
∴ The number of triangles formed
= `""^12"C"_3 - ""^4"C"_3`
= `220 - (4 xx 3 xx 2)/(3 xx 2)`
= 220 – 4
= 216
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