Obtuse-Angled Triangles Count

CAT 2008 Slot 1 · QA · Medium · Geometry

Consider obtuse-angled triangles with sides 8 cm8\text{ cm}, 15 cm15\text{ cm} and x cmx\text{ cm}. If xx is an integer, then how many such triangles exist?

  1. A.

    5

  2. B.

    21

  3. C.

    10

  4. D.

    15

  5. E.

    14

Answer

C

Explanation

By triangle inequality, 158<x<15+8    7<x<23|15 - 8| < x < 15 + 8 \implies 7 < x < 23.

For the triangle to be obtuse, the square of the longest side must be strictly greater than the sum of squares of the other two sides.

Case 1: 1515 is the longest side (x15x \le 15): 152>82+x2    225>64+x2    x2<16115^2 > 8^2 + x^2 \implies 225 > 64 + x^2 \implies x^2 < 161 Since x>7x > 7, integer values of xx can be 8,9,10,11,128, 9, 10, 11, 12. Total = 5 values.

Case 2: xx is the longest side (x>15x > 15): x2>82+152    x2>64+225=289    x>17x^2 > 8^2 + 15^2 \implies x^2 > 64 + 225 = 289 \implies x > 17 Since x<23x < 23, integer values of xx can be 18,19,20,21,2218, 19, 20, 21, 22. Total = 5 values.

Total possible integer values for x=5+5=10x = 5 + 5 = 10.

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