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NDA 2024 · PYQ · Trigonometry / Triangles · medium

Consider the following statements: 1. In a triangle ABC, if cotA·cotB·cotC > 0, then the triangle is an acute angled triangle. 2. In a triangle ABC, if tanA·tanB·tanC > 0, then the triangle is an obtuse angled triangle. Which of the above statements is/are correct?

  1. A.1 only✓ Correct
  2. B.2 only
  3. C.Both 1 and 2
  4. D.Neither 1 nor 2

Explanation

In an acute triangle, all angles are < 90°, so all cotangents are positive, and cotA·cotB·cotC > 0. Conversely if cotA·cotB·cotC > 0, then either all three cotangents are positive (acute triangle) or two are negative (impossible in a triangle where at most one angle exceeds 90°). So statement 1 is correct. For statement 2: in any triangle, tanA + tanB + tanC = tanA·tanB·tanC. In an acute triangle, all tangents are positive, so product > 0. So tanA·tanB·tanC > 0 implies acute triangle, not obtuse. Statement 2 is incorrect.
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