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A student claims to have drawn a regular polygon each of whose interior angles measures exactly 155°. Why is that impossible?

Competition practiceGeometry
  • AAn interior angle of a regular polygon can never be larger than 150°
  • BThe matching exterior angle is 25°, and 360° is not exactly divisible by 25°
  • CThe interior angle of a regular polygon must always be a whole multiple of 30°
  • DAn exterior angle as small as 25° is below the smallest one that any regular polygon can have

Step-by-Step Solution

At each vertex the interior and exterior angles are supplementary, so the exterior angle would be 180° − 155° = 25°. The exterior angles of any polygon add to exactly 360°, so a regular n-sided polygon needs 360/n = 25, giving n = 14.4 — not a whole number, so no such polygon exists. The other explanations are all false: a regular 24-gon has interior angles of 165°, well above 150°; a regular pentagon has 108°, which is no multiple of 30°; and exterior angles can be made as small as one likes by taking enough sides, a regular 360-gon having exterior angles of 1°.
Answer: B) The matching exterior angle is 25°, and 360° is not exactly divisible by 25°

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