Fundamentholfundamenthol
JEE Advanced2023Paper 1MATH-II
Q.

Let . Three distinct points , and are randomly chosen from . Then the probability that , and form a triangle whose area is a positive integer, is

  1. A

  2. B

  3. C

  4. D

Solution

The two inequalities intersect on the boundary at , , and the ellipse meets the -axis at . Enumerating integer points satisfying both strict inequalities, the candidates fall on the vertical lines and :

: (five points).

: (seven points).

So and the sample space size is .

Every triangle formed by three of these points has two vertices on a common vertical line or (since any three points include at least two on the same line by pigeonhole). Take the base on that line; the third vertex lies on the other line, so the perpendicular distance (horizontal) is . Hence the area equals , which is a positive integer iff the base length is an even positive integer.

Count triangles by even base lengths (with base on or and third vertex on the opposite line):

Base : on there are such pairs and on there are ; combined with or choices of the third vertex, gives .

Base : .

Base : .

Total favourable triangles .

Required probability .

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