 ## Inscribed Rectangle

#### Problem

A rectangle and an equilateral triangle with equal area are inscribed in a circle with radius, 2. Find the dimensions of the rectangle.

#### Solution

By drawing a diagonal in the rectangle, we form a diameter. In the case of the triangle, we use the geometrical result which states that the centre of an inscribed equilateral triangle is 1/3 the height of the triangle. Hence the radius is 2/3 the height of the triangle, and we deduce that the height of the triangle is 3. Applying the Pythagorean theorem to the rectangle, x2 + y2 = 16   .

Similarly with the triangle, 4a2 = a2 + 9, so a = 3.

Hence the area of the triangle is 3 3.

As the areas are equal, the area of the rectangle, xy = 3 3,
so x2y2 = 27   .

From  we get, y2 = 16 x2.

Substituting this into  we get, x2(16 x2) = 27.

Leading to the quartic, x4 16x2 + 27 = 0.

Solving this as a quadratic in x2 we obtain the roots, x2 = 8  37.

Using y2 = 16 x2 we observe that y2 = 16 (8  37).
That is, when x2 = 8 + 37, y2 = 8  37, and vice versa.

Therefore the rectangle has dimensions, (8 + 37) by (8  37).

Can you generalise for a circle with radius, r?

Find the side length of the triangle if the rectangle measures, x by y.

What if the triangle has side length, a?

Problem ID: 63 (Jan 2002)     Difficulty: 3 Star

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