Medium MCQ +4 / -1 PYQ · JEE Mains 2021

For real numbers $\alpha$ and $\beta$ $\ne$ 0, if the point of intersection of the straight lines

${{x - \alpha } \over 1} = {{y - 1} \over 2} = {{z - 1} \over 3}$ and ${{x - 4} \over \beta } = {{y - 6} \over 3} = {{z - 7} \over 3}$, lies on the plane x + 2y $-$ z = 8, then $\alpha$ $-$ $\beta$ is equal to :

  1. A 5
  2. B 9
  3. C 3
  4. D 7 Correct answer

Solution

First line is ($\phi$ + $\alpha$, 2$\phi$ + 1, 3$\phi$ + 1)<br><br>and second line is (q$\beta$ + 4, 3q + 6, 3q + 7)<br><br>For intersection<br><br>$\phi$ + $\alpha$ = q$\beta$ + 4 ...... (i)<br><br>2$\phi$ + 1 = 3q + 6 .... (ii)<br><br>3$\phi$ + 1 = 3q + 7 ...... (iii)<br><br>for (ii) &amp; (iii) $\phi$ = 1, q = $-$1<br><br>So, from (i) $\alpha$ + $\beta$ = 3<br><br>Now, point of intersection is ($\alpha$ + 1, 3, 4)<br><br>It lies on the plane.<br><br>Hence, $\alpha$ = 5 &amp; $\beta$ = $-$2

About this question

Subject: Mathematics · Chapter: Three Dimensional Geometry · Topic: Direction Cosines and Ratios

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