Angles' ratio

Level pending

Imagine two regular polygons,one with 101 101 sides, and the other 102 102 sides. If the ratio of interior angle for Polygon101 to interior angle for Polygon102 = x : y x:y , what is the value of x x if the ratio was in simplest form?


The answer is 5049.

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1 solution

Cl Chong
Nov 8, 2019

I found 2 ways to solve this problem.

Way 1:

n n = number of sides

α α = interior angle

n × ( π α ) = 2 π n \times (π-α) = 2π

α = π α = π- 2 π n \frac{2π}{n} = π = π ( 1 1- 2 n \frac{2}{n} ) = π = π ( n 2 n \frac{n-2}{n} ) = = π n \frac{π}{n} ( n 2 ) (n-2)

α 102 α 101 \frac{α_{102}}{α_{101}} = ( 102 2 ) / 102 ( 101 2 ) / 101 \frac{(102-2)/102}{(101-2)/101} = 50 51 \frac{50}{51} × \times 101 99 \frac{101}{99} = 5050 5049 \frac{5050}{\boxed{5049}}

Way 2:

There is actually a pattern!

Polygon's number of sides 3,4 4,5 5,6
Term x 2 5 9
Term y 3 6 10

Formulas:

First polygon's number of sides = f f

Term y y = ( f 1 ) × ( f ) 2 \frac{(f-1) \times (f)}{2}

{**Note that this is also the formula for triangular number!! where n = (f-1); refer to https://en.wikipedia.org/wiki/Triangular_number }

Term x x = Term y y - 1

Use them:

Term y y = 100 × 101 2 \frac{100 \times 101}{2} = 10100 2 \frac{10100}{2} = 5050 5050

Term x x = 5050 1 5050-1 = 5049 \boxed{5049}

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