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Vector Algebra question

2024 · 1 Feb · Shift 1 · Q38
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  5. /2024 · 1 Feb · Shift 1 · Q38

Vector Algebra question

2024 · 1 Feb · Shift 1 · Q38

JEE MainMathematicsVector AlgebraMCQ+4 / −1
Let a→=−5i^+j^−3k^,b→=i^+2j^−4k^\overrightarrow{\mathrm{a}}=-5 \hat{i}+\hat{j}-3 \hat{k}, \overrightarrow{\mathrm{b}}=\hat{i}+2 \hat{j}-4 \hat{k}a=−5i^+j^​−3k^,b=i^+2j^​−4k^ and c→=(((a→×b→)×i^)×i^)×i^\overrightarrow{\mathrm{c}}=(((\overrightarrow{\mathrm{a}} \times \overrightarrow{\mathrm{b}}) \times \hat{i}) \times \hat{i}) \times \hat{i}c=(((a×b)×i^)×i^)×i^. Then c⃗⋅(−i^+j^+k^)\vec{c} \cdot(-\hat{i}+\hat{j}+\hat{k})c⋅(−i^+j^​+k^) is equal to :
  1. A
    -12
  2. B
    -10
  3. C
    -13
  4. D
    -15
View written solutionFree

Correct answer: A

  1. Given vectors
a⃗=−5i^+j^−3k^,b⃗=i^+2j^−4k^\vec a = -5\hat i + \hat j - 3\hat k, \qquad \vec b = \hat i + 2\hat j - 4\hat ka=−5i^+j^​−3k^,b=i^+2j^​−4k^

We need

c⃗=((((a⃗×b⃗)×i^)×i^)×i^)\vec c = ((((\vec a \times \vec b) \times \hat i) \times \hat i) \times \hat i)c=((((a×b)×i^)×i^)×i^)

and then compute

c⃗⋅(−i^+j^+k^).\vec c \cdot (-\hat i + \hat j + \hat k).c⋅(−i^+j^​+k^).
  1. Find a⃗×b⃗\vec a \times \vec ba×b

Using the determinant:

a⃗×b⃗=∣i^j^k^−51−312−4∣\vec a \times \vec b= \begin{vmatrix} \hat i & \hat j & \hat k \\ -5 & 1 & -3 \\ 1 & 2 & -4 \end{vmatrix}a×b=​i^−51​j^​12​k^−3−4​​ =i^ (1⋅(−4)−(−3)⋅2)−j^ ((−5)⋅(−4)−(−3)⋅1)+k^ ((−5)⋅2−1⋅1)= \hat i\,(1\cdot(-4)-(-3)\cdot2) - \hat j\,((-5)\cdot(-4)-(-3)\cdot1) + \hat k\,((-5)\cdot2-1\cdot1)=i^(1⋅(−4)−(−3)⋅2)−j^​((−5)⋅(−4)−(−3)⋅1)+k^((−5)⋅2−1⋅1) =i^(−4+6)−j^(20+3)+k^(−10−1)= \hat i(-4+6) - \hat j(20+3) + \hat k(-10-1)=i^(−4+6)−j^​(20+3)+k^(−10−1) =2i^−23j^−11k^= 2\hat i - 23\hat j - 11\hat k=2i^−23j^​−11k^

So,

p⃗=a⃗×b⃗=2i^−23j^−11k^.\vec p = \vec a \times \vec b = 2\hat i - 23\hat j - 11\hat k.p​=a×b=2i^−23j^​−11k^.
  1. Compute successive cross products with i^\hat ii^

Let

p⃗=(2,−23,−11).\vec p = (2,-23,-11).p​=(2,−23,−11).

For any vector (x,y,z)(x,y,z)(x,y,z),

(x,y,z)×i^=(0,z,−y).(x,y,z) \times \hat i = (0,z,-y).(x,y,z)×i^=(0,z,−y).

First cross product

p⃗×i^=(0,−11,23)\vec p \times \hat i = (0,-11,23)p​×i^=(0,−11,23)

So,

p⃗1=−11j^+23k^.\vec p_1 = -11\hat j + 23\hat k.p​1​=−11j^​+23k^.

Second cross product

p⃗1×i^=(0,23,11)\vec p_1 \times \hat i = (0,23,11)p​1​×i^=(0,23,11)

So,

p⃗2=23j^+11k^.\vec p_2 = 23\hat j + 11\hat k.p​2​=23j^​+11k^.

Third cross product

p⃗2×i^=(0,11,−23)\vec p_2 \times \hat i = (0,11,-23)p​2​×i^=(0,11,−23)

Hence,

c⃗=11j^−23k^.\vec c = 11\hat j - 23\hat k.c=11j^​−23k^.
  1. Now compute the dot product
c⃗⋅(−i^+j^+k^)=(0,11,−23)⋅(−1,1,1)\vec c \cdot (-\hat i + \hat j + \hat k) = (0,11,-23)\cdot(-1,1,1)c⋅(−i^+j^​+k^)=(0,11,−23)⋅(−1,1,1) =0⋅(−1)+11⋅1+(−23)⋅1= 0\cdot(-1) + 11\cdot1 + (-23)\cdot1=0⋅(−1)+11⋅1+(−23)⋅1 =11−23=−12= 11 - 23 = -12=11−23=−12
  1. Compare with options

The value is

−12\boxed{-12}−12​

So the correct option is:

A.

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