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Rotating Triangle

What happens to the perimeter of triangle ABC as the two smaller circles change size and roll around inside the bigger circle?

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Two semicircle sit on the diameter of a semicircle centre O of twice their radius. Lines through O divide the perimeter into two parts. What can you say about the lengths of these two parts?

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Show that for any triangle it is always possible to construct 3 touching circles with centres at the vertices. Is it possible to construct touching circles centred at the vertices of any polygon?


Stage: 4 Challenge Level: Challenge Level:3 Challenge Level:3 Challenge Level:3

The second part of this question about the scalene triangle remains a Tough Nut but Vassil, from Lawnswood High School, Leeds sent a good solution to the first part.

Vassils' diagram shows that the areas of the purple red and yellow triangles, for any position of $P$, add up to the area of the whole equilateral triangle. Taking the length of the side of the equilateral triangle to be $a$ units this gives: $${ah_1\over 2} + {ah_2\over 2} + {ah_3\over 2} = {ah\over 2}.$$ Hence $$h_1 + h_2 + h_3 = h = \rm{constant}.$$

The sum of the perpendicular distances from $P$ to the sides of the triangle is equal to the height of the triangle.