A regular polygon will be drawn with a number of sides randomly chosen from the whole numbers 3 through 10. What is the probability that the internal angles will be greater than 110 ?
A) 1/8
B) 1/4
C) 3/8
D) 1/2
E) 5/8
Angle at any vertice of the n sided polygon = \(\frac{180(n - 2)}{n}\)
As we know that upto a regular polygon of 5 side the angle is < 110.
From sides 6 to 10(total 5) have angle > 110
Angle at any vertice of the 6 sided polygon = \(\frac{180(6 - 2)}{6}\) = 120
Hence \(probability = \frac{required possibilities }{ total possibilities }\)
= 5/8
Two things to observe:
1. If you know how regular polygon works then you know that any regular polygon having 6 or more sides would have angle > 110. Visualise it.
2. If you can't visualise it, know from the formula \(\frac{180(n - 2)}{n}\) that numerator would increase in the multiples of 180 whereas denominator would increase by 1 only. Thus, if 6 sided regular polygon has angle > 110, others with more than 6 sides definitely would have angle more than 110.
Answer E.