Practicing Success

Target Exam

CUET

Subject

-- Mathematics - Section B1

Chapter

Vectors

Question:

Statement-1: Any vector in space can be uniquely written as the linear combination of three non-coplanar vectors.

Statement-2: If $\vec a,\vec b,\vec c$ are three non-coplanar vectors and r is any vector in space, then 

$[\vec a\,\,\vec b\,\,\vec r]\vec c+[\vec b\,\,\vec c\,\,\vec r]\vec a+[\vec c\,\,\vec a\,\,\vec r]\vec b=[\vec a\,\,\vec b\,\,\vec c]\vec r$

Options:

Statement-1 is True, Statement-2 is True; Statement-2 is a correct explanation for Statement-1.

Statement-1 is True, Statement-2 is True; Statement-2 is not a correct explanation for Statement-1.

Statement-1 is True, Statement-2 is False.

Statement-1 is False, Statement-2 is True.

Correct Answer:

Statement-1 is True, Statement-2 is True; Statement-2 is not a correct explanation for Statement-1.

Explanation:

Clearly, statement-1 is true.

We have, 

$\vec r = x\vec a+y\vec b +z\vec c$   ...(i)

Taking product successively with $\vec b ×\vec c, \vec c×\vec a$ and $\vec a×\vec b$, we obtain

$x=\frac{[\vec b\,\,\vec c\,\,\vec r]}{[\vec a\,\,\vec b\,\,\vec c]},y=\frac{[\vec c\,\,\vec a\,\,\vec r]}{[\vec a\,\,\vec b\,\,\vec c]}$ and $z=\frac{\vec a\,\,\vec b\,\,[\vec r]}{[\vec a\,\,\vec b\,\,\vec c]}$

Substituting the values of x, y, z in (i), we get

$[\vec a\,\,\vec b\,\,\vec r]\vec c+[\vec b\,\,\vec c\,\,\vec r]\vec a+[\vec c\,\,\vec a\,\,\vec r]\vec b=[\vec a\,\,\vec b\,\,\vec c]\vec r$

So, statement-2 is true. But, statement-2 is not a correct explanation for statement-1.