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JEE Main 20238 AprilMorning Shift
Exam
JEE Main
Year
2023
Date
8 April
Shift
Morning Shift
Subject
Mathematics
Chapter
Vector Algebra

JEE Main 2023 · 8 April · Morning Shift · Mathematics · Vector Algebra

If the points with position vectors α ^ i + 10 ^ j + 13 ^ k , 6 ^ i...

If the points with position vectors \alpha\overset{^}{i} + 10\overset{^}{j} + 13\overset{^}{k}, 6\overset{^}{i} + 11\overset{^}{j} + 11\overset{^}{k}, \dfrac{9}{2}\overset{^}{i} + \beta\overset{^}{j} - 8\overset{^}{k} are collinear, then ${\left(19\alpha - 6\beta\right)}^{2} is equal toIf the points with position vectors α ^ i + 10 ^ j + 13 ^ k , 6 ^ i + 11 ^ j + 11 ^ k , (9)/(2) ^ i + β ^ j - 8 ^ k are

Options

  1. A3636
  2. B2525
  3. C4949
  4. D1616

Correct answer

A. 3636

Step-by-step solution

The given position vectors can be written as

  • A(α,10,13)A ( \alpha, 10, 13 )
  • B(6,11,11)B ( 6, 11, 11 )
  • C(92,β,−8)C\left(\dfrac{9}{2}, \beta, - 8\right)

Also given that these three points are collinear.

Let us assume that the point BB divides AB,BCAB, BC in the ratio k:1k: 1.

Since, A,B,CA, B, C are collinear

If the points with position vectors α ^ i + 10 ^ j + 13 ^ k , 6 ^ i + 11 ^ j + 11 ^ k , (9)/(2) ^ i + β ^ j - 8 ^ k are

On applying section formula for zz co-ordinate we get,

11=−8k+13k+111 = \dfrac{- 8k + 13}{k + 1}

⇒11k+11=−8k+13⇒ 11k + 11 = - 8k + 13

⇒19k=2⇒ 19k = 2

⇒k=219⇒ k = \dfrac{2}{19}

∴∴Ratio=2:19= 2: 19

Now, α×19+(92)×22+19=6\dfrac{\alpha \times 19 + \left(\dfrac{9}{2}\right) \times 2}{2 + 19} = 6

⇒19α=117⇒ 19\alpha = 117

⇒α=11719⇒ \alpha = \dfrac{117}{19}

Now similarly, 2β+19021=11\dfrac{2\beta + 190}{21} = 11

⇒β=412⇒ \beta = \dfrac{41}{2}

∴(19α−6β)2=(117−123)2=36∴ {\left(19\alpha - 6\beta\right)}^{2} = {\left(117 - 123\right)}^{2} = 36

Hence this is the correct option.

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