JEE Advanced · Prototype 1.1
Turn coplanarity into an equation
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Question 1.1
A = (1, 2, 3), B = (2, −1, 1), and C = (λ, 3, 4) are coplanar. Find λ.
Visual explanationSee why the answer works⌄
A · (B × C) = 0
Coplanarity requires A · (B × C) = 0. Expanding the determinant gives 5λ − 5 = 0, so λ = 1.