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Bunuel
A triangle has sides of length 12, 15 and k units. What is the length of the longest side?

(1) The perimeter of the triangle is a multiple of 9.
(2) One of the interior angles of the triangle is equal to the sum of the other two interior angles.


Solution


Step 1: Analyse Question Stem


    • 12, 15 and k units are sides of a triangle.
      o [We don’t know whether k is integer or not.]
    • Using the side properties of a triangle, we can write,
      o \(15 – 12 < k < 12 + 15 ⟹ 3 < k < 27 ……(i)\)
    • We need to find the length of the longest side.
      o If we know k we can compare 15 and k and decide which side is the longest.

Step 2: Analyse Statements Independently (And eliminate options) – AD/BCE


Statement 1: The perimeter of the triangle is a multiple of 9.
    • From (i) we have, \(12 + 15 + 3 < 12 + 15 + k < 12 + 15 + 27 ⟹ 30 < 9n < 54\), where n is a positive integer.
      o So, n can be either 4 or 5.
         If \(n = 4 \) then \(k = 9*4 – 12 – 15 = 9\)
          • In this case the longest side is 15
         If \(n = 5\) then \(k = 9*5 – 12 – 15 = 18\)
          • In this case the longest side is 18.
          • We are getting two different results.
Hence, statement 1 is NOT sufficient and we can eliminate answer Options A and D.
Statement 2: One of the interior angles of the triangle is equal to the sum of the other two interior angles.
    • Let us assume that the three interior angles of the triangle are a, b, and c.
      o \( a+b = c \) and \(a+b+c = 180\) degrees \(⟹2c = 180 ⟹c = 90\) degrees.
      o So, the given triangle is right angled triangle. However, from this information we cannot decide which one is longest, either one of 15 or k can be longest side.
Hence, statement 2 is also NOT sufficient and we can eliminate answer Option B.

Step 3: Analyse Statements by combining.


    • From statement 1: k can be 9 or 18
    • From statement 2: Given triangle is a right- angled triangle. So,
      o If k is the longest side then \(k^2 = 12^2 + 15^2 ⟹ k = \sqrt{369}\)
      o If 15 is the longest then \(k^2 = 15^2 – 12^2 ⟹ k = 9\)
    • On combining both statements, we get, k \(= 9\)
      o Hence, the longest side \(= 15\)
Thus, the correct answer is Option C.
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With A definitely with figure out clearly insufficient 18,9 are possiblities however when we are going towards we can figure out that it produce a right angled traingle there in we get 9 and 3*(41)^1//2 combining both these we get C therefore IMO C
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