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BITSAT 2019
Exam
BITSAT
Year
2019
Subject
Mathematics
Chapter
Vector Algebra

BITSAT 2019 · Mathematics · Vector Algebra

If → a , → b , → c are non-coplaner vectors such that → b × → c = →...

If a→,b→,c→\overset{→}{a}, \overset{→}{b}, \overset{→}{c} are non-coplaner vectors such that b→×c→=a→;c→×a→=b→;a→×b→=c→\overset{→}{b} \times \overset{→}{c} = \overset{→}{a}; \overset{→}{c} \times \overset{→}{a} = \overset{→}{b}; \overset{→}{a} \times \overset{→}{b} = \overset{→}{c},& then which of the following is not TRUE?

Options

  1. A∣a→∣−∣b→∣=0| \overset{→}{a} | - | \overset{→}{b} | = 0
  2. B∣a→∣=∣b→∣=∣c→∣=2| \overset{→}{a} | = | \overset{→}{b} | = | \overset{→}{c} | = 2
  3. C\left[\begin{cases}\overset{→}{a} & \overset{→}{b} & \overset{→}{c}\end{cases}\right] = 1
  4. D∣a→∣∣b→∣∣c→∣=1| \overset{→}{a} | | \overset{→}{b} | | \overset{→}{c} | = 1

Correct answer

B. ∣a→∣=∣b→∣=∣c→∣=2| \overset{→}{a} | = | \overset{→}{b} | = | \overset{→}{c} | = 2

Step-by-step solution

Given,&

a→×b→=c→\overset{→}{a} \times \overset{→}{b} = \overset{→}{c}

b→×c→=a→\overset{→}{b} \times \overset{→}{c} = \overset{→}{a}

c→×a→=b→\overset{→}{c} \times \overset{→}{a} = \overset{→}{b}

(b→×c→)⋅a→=a→⋅a→( \overset{→}{b} \times \overset{→}{c} ) \cdot \overset{→}{a} = \overset{→}{a} \cdot \overset{→}{a}

(c→×a→)⋅b→=b→⋅b→( \overset{→}{c} \times \overset{→}{a} ) \cdot \overset{→}{b} = \overset{→}{b} \cdot \overset{→}{b}

a→2−b→2=(b→×c→)⋅a→−(c→×a→)⋅b→\overset{→}{a}2 - \overset{→}{b}2 = ( \overset{→}{b} \times \overset{→}{c} ) \cdot \overset{→}{a} - ( \overset{→}{c} \times \overset{→}{a} ) \cdot \overset{→}{b}

=[= [\begin{cases}\overset{→}{c} & \overset{→}{a} & \overset{→}{b}\end{cases}]−[] - [\begin{cases}\overset{→}{b} & \overset{→}{c} & \overset{→}{a}\end{cases}]=0] = 0

∴∣a→∣2−∣b→∣2=0∴ | \overset{→}{a}{|}^{2} - | \overset{→}{b}{|}^{2} = 0

∣a→∣−∣b→∣=0| \overset{→}{a} | - | \overset{→}{b} | = 0

⇒∣a→∣=∣b→∣⇒ | \overset{→}{a} | = | \overset{→}{b} |

We know that,~

\left[\overset{→}{a} \times \overset{→}{b}\overset{→}{b} \times \overset{→}{c}\overset{→}{c} \times \overset{→}{a}\right] = [ \overset{→}{a}\overset{→}{b}\overset{→}{c}{]}^{2}</p><p>\left[a→b→c→\begin{array}{ll}\overset{→}{a} \overset{→}{b} \overset{→}{c}\end{array}$\right] = \left[\begin{cases}\overset{→}{a} & \overset{→}{b} & \overset{→}{c}\end{cases}\right]2</p><p></p><p>⇒ \left[{a→b→c→\begin{cases}\overset{→}{a} & \overset{→}{b} & \overset{→}{c}\end{cases}\right]=1= 1

∣a→∣∣b→∣∣c→∣=1| \overset{→}{a} | | \overset{→}{b} | | \overset{→}{c} | = 1

∴∣a→∣=∣b→∣=∣c→∣≠2∴ | \overset{→}{a} | = | \overset{→}{b} | = | \overset{→}{c} | \ne 2

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