String of Squares

Geometry Level 4

A big mobile is outlined from an isosceles triangle A B C ABC , where A B = 70 cm AB = 70\text{ cm} . and A C = B C = 125 cm AC = BC = 125\text{ cm} .

First, the inscribed circles are drawn recursively touching one another endlessly towards to the vertex C C before the blue squares are inscribed in those circles along the height from vertex C C to the base. Finally, a pink square of 1 cm 2 1\text{ cm}^2 is attached to the vertex C C outside the triangle for decoration.

What is the total area of all colored squares (in cm 2 \text{cm}^2 ).


The answer is 2017.

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1 solution

By Pythagorean theorem, the height from vertex C C to base A B AB = 12 5 2 3 5 2 = 120 \sqrt{125^2 - 35^2} = 120

A radius of an inscribed circle inside a triangle = 2 × Area of triangle perimeter = 70 × 120 70 + 125 + 125 = 8400 320 = 105 4 \dfrac{2\times \text{Area of triangle}}{\text{perimeter}} = \dfrac{70\times 120}{70+125+125} = \dfrac{8400}{320} = \dfrac{105}{4} .

Therefore, the biggest incircle's diameter = 105 2 = 52.5 \dfrac{105}{2} = 52.5

Then for the next second incircle, the new height will be 120 52.5 = 67.5 120 - 52.5 = 67.5 . Thus, the ratio of the similar smaller triangle to the original one = 67.5 120 = 9 16 \dfrac{67.5}{120} =\dfrac{9}{16} .

Because of similarity, the ratio for the next recursive incircle will always be 9 16 \dfrac{9}{16} , and so it is a geometric series for the blue squares with each square's area = d 2 2 \dfrac{d^2}{2} where d d is the diameter of the incircle or the diagonal of the square. It should be noted that the ratio of the next recursive square is ( 9 16 ) 2 (\dfrac{9}{16})^2 as two sides are multiplied.

Thus, the total blue squares' area = 1 2 ( 52. 5 2 1 ( 9 16 ) 2 ) = 52. 5 2 2 × 256 175 = 2016 \dfrac{1}{2}(\dfrac{52.5^2}{1 - (\dfrac{9}{16})^2}) = \dfrac{52.5^2}{2}\times \dfrac{256}{175} = 2016 .

Now don't forget the little last pink unit square. The total colored area will then equal 2016 + 1 = 2017 2016+1 = \boxed{2017} .

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