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 Same area (Posted on 2016-03-15)
Given an equilateral triangle inscribed in a circle with a radius R.

Evaluate the dimensions (height, width) of an inscribed rectangle with the same area as the triangle.

 See The Solution Submitted by Ady TZIDON No Rating

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 Solution Comment 2 of 2 |
WLOG let R=1

Then the triangle has an altitude of 3/2, edges of sqrt(3) and area of sqrt(27)/4.

Let L and W be the sides of the rectangle.  The rectangle has area L*W and an diagonal of 2, which forms a right triangle with a pair of sides.  Then:
L*W = sqrt(27)/4
L^2 + W^2 = 4

Substituting and simplifying yields:
L^4 - 4L^2 + 27/16 = 0

The two positive roots of the quartic equation are the dimensions of the rectangle: sqrt[8+sqrt(37)]/2 and sqrt[8-sqrt(37)]/2.

Then for general radius R: L = R*sqrt[8+sqrt(37)]/2 and W = R*sqrt[8-sqrt(37)]/2

 Posted by Brian Smith on 2016-03-16 12:17:27

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