Complex Polygon

Algebra Level 4

On the complex plane, a regular polygon formed by connecting each consecutive point that is a solution to the equation x n = 1 , x^n = 1, is centered at the origin and has a vertex at z = ( 3 2 + i 2 ) . z = \bigg(\frac{\sqrt{3}}{2} + \frac{i}{2}\bigg). Let A be the minimum possible number of sides found on this polygon, and let B be area of the polygon with A sides. Find the value of A + + B

Note:

You may use a calculator to evaluate the area of the polygon
i i is the imaginary unit


The answer is 15.

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1 solution

Akeel Howell
Jun 11, 2016

The point z = ( 3 2 + i 2 ) z = \bigg(\frac{\sqrt{3}}{2} + \frac{i}{2}\bigg) is on a regular polygon centered at the origin. This tells us that the polygon is inscribed in a circle of radius 1 1 centered at the origin, and z = ( 3 2 + i 2 ) z = \bigg(\frac{\sqrt{3}}{2} + \frac{i}{2}\bigg) is a point on the circle in the complex plane. Thus, x = 3 2 = cos 3 0 x = \frac{\sqrt{3}}{2} = \cos 30^\circ and y = 1 2 i = i sin 3 0 y = \frac{1}{2}i = i\sin 30^\circ . . . \therefore The angle between the points 1 1 , 0 0 and ( 3 2 + i 2 ) \bigg(\frac{\sqrt{3}}{2} + \frac{i}{2}\bigg) is 3 0 30^\circ . So each point on the polygon is separated by 3 0 30^\circ of arc on the unit circle. Thus, the polygon has ( 36 0 3 0 ) = 12 \biggl(\frac{360^\circ}{30^\circ}\biggr) = 12 sides. Therefore, the minimum possible number of sides the polygon has is 12. 12. . . So A = 12 = 12 . The area of the dodecadgon ( 12 12 sides) is: A = r 2 12 sin ( 9 0 18 0 12 ) cos ( 9 0 18 0 12 ) . {A}_{\circ} = r^2 12\sin\bigg(90^\circ - \frac{180^\circ}{12}\bigg) \bullet \cos\bigg(90^\circ - \frac{180^\circ}{12}\bigg). A = 12 sin ( 7 5 ) cos ( 7 5 ) = 12 4 = 3. {A}_{\circ} = 12\sin\big(75^\circ\big)\cos\big(75^\circ\big) = \frac{12}{4} = 3. So A = 12 = 12 and B = 3 = 3 . . \therefore A + + B = 15 . = \fbox{15}.

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