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JEE Main 2020
Matrices & Determinants
Matrices and Determinants
Medium

Question

Let A = {X = (x, y, z) T : PX = 0 and x 2 + y 2 + z 2 = 1} where P = \left[ {\matrix{ 1 & 2 & 1 \cr { - 2} & 3 & { - 4} \cr 1 & 9 & { - 1} \cr } } \right], then the set A :

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Solution

Key Concepts

This problem involves solving a system of homogeneous linear equations represented by a matrix equation PX=0PX=0, along with an additional constraint x2+y2+z2=1x^2 + y^2 + z^2 = 1.

  1. Homogeneous System of Linear Equations: For a system PX=0PX=0 where PP is an n×nn \times n matrix, the nature of solutions depends on the determinant of PP:
    • If det(P)0\det(P) \neq 0, the system has only the trivial solution X=0X = \mathbf{0} (i.e., x=y=z=0x=y=z=0).
    • If det(P)=0\det(P) = 0, the system has non-trivial solutions (inf

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