UPSC Prelims 2015·CSAT·Logical Reasoning·Deductive Logic

In a plane, line X is perpendicular to line Y and parallel to line Z; line U is perpendicular to both lines V and W; line X is perpendicular to line V. Which one of the following statements is correct?

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  1. AZ, U and W are parallel
  2. BX, V and Y are parallel
  3. CZ, V and U are all perpendicular to W
  4. DY, V and W are parallelCorrect

Explanation

Let's break down the given information and deduce relationships between the lines: Given conditions: 1. Line X is perpendicular to line Y (X _|_ Y). 2. Line X is parallel to line Z (X || Z). 3. Line U is perpendicular to line V (U _|_ V). 4. Line U is perpendicular to line W (U _|_ W). 5. Line X is perpendicular to line V (X _|_ V). Deductions: From (1) X _|_ Y and (2) X || Z: * If X is perpendicular to Y, and Z is parallel to X, then Z must also be perpendicular to Y (Z _|_ Y). From (3) U _|_ V and (4) U _|_ W: * If two lines (V and W) are both perpendicular to a third line (U) in the same plane, then the two lines (V and W) must be parallel to each other (V || W). From (1) X _|_ Y and (5) X _|_ V: * If two lines (Y and V) are both perpendicular to a third line (X) in the same plane, then the two lines (Y and V) must be parallel to each other (Y || V). Now let's evaluate the options: A) Z, U and W are parallel We know U _|_ W. This means U and W are not parallel. So, statement A is incorrect. B) X, V and Y are parallel We know X _|_ Y. This means X and Y are not parallel. So, statement B is incorrect. C) Z, V and U are all perpendicular to W We deduced V || W. If V and W are parallel, they cannot be perpendicular (unless they are the same line, which is not implied). So, V is not perpendicular to W. Thus, statement C is incorrect. D) Y, V and W are parallel From our deductions: * We found Y || V. * We found V || W. Combining these, if Y is parallel to V, and V is parallel to W, then Y, V, and W must all be parallel to each other (Y || V || W). This statement is correct. The final answer is D
Logical Reasoning: In a plane, line X is perpendicular to line Y and parallel to line Z; line U is perpendicular to both lines V and W; lin

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