A regular polygon inscribed in a unit circle has a side length of
Find the number of sides of the polygon.
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Let the angle between 2 radii joining the centre of the circle to 2 adjacent vertices of the polygon be x.
Applying the cosine rule we get: ( s i d e l e n g t h ) 2 = 1 2 + 1 2 − 2 ( 1 ) ( 1 ) c o s ( x )
( 2 − 2 + 2 + 2 + 3 ) 2 = 2 - 2 cos(x)
( 2 − 2 + 2 + 2 + 3 ) = 2 - 2 cos (x)
2 cos (x) = ( 2 + 2 + 2 + 3 )
Squaring both sides we get:
4 c o s 2 ( x ) = ( 2 + 2 + 2 + 3 )
4 c o s 2 ( x ) - 2 = ( 2 + 2 + 3 )
2 cos (2x) = ( 2 + 2 + 3 )
Squaring both sides again we get:
4 c o s 2 ( 2 x ) = 2 + 2 + 3
4 c o s 2 ( 2 x ) - 2 = 2 + 3
2 cos (4x) = 2 + 3
Squaring both sides again we get:
4 c o s 2 ( 4 x ) = 2 + 3
4 c o s 2 ( 4 x ) - 2 = 3
2 cos (8x) = 3
cos (8x) = 2 1 3
8x = 30 degrees
x = 30/8 degrees
Number of sides = 3 0 / 8 3 6 0 = 3 0 ( 3 6 0 ) ( 8 ) = 96 sides
A n s w e r = 9 6