Extension!

Geometry Level pending

Let m > 1 m > 1 .

In right A B C \triangle{ABC} with A B = m \overline{AB} =m , B D \overline{BD} and B C \overline{BC} are tangent to the two inscribed unit semicircles at F F and H H respectively.

Extend the above diagram to n n congruent unit circles as follows:

Draw another unit semicircle through C and construct the tangent from B and mark the intersection with the extended line AC and continue this process for the n n unit semicircles.

(1) Find a formula for the area of the right triangle formed which contains the n n unit semicircles.

(2) Using m = 2 m = 2 and n = 10 n = 10 write a program to compute the area of A n A_{n} .

Below is a diagram for the area of A 4 A_{4}


The answer is 2473.00493680999.

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1 solution

Rocco Dalto
Mar 14, 2021

Let m > 1 m > 1 .

A B D E F D m = B D y 1 B D = m y 1 \triangle{ABD} \sim \triangle{EFD} \implies m = \dfrac{\overline{BD}}{y_{1}} \implies \overline{BD} = my_{1}

Using the Pythagorean theorem on A B D m 2 y 1 2 = m 2 + ( 1 + y 1 ) 2 \triangle{ABD} \implies m^2y_{1}^2 = m^2 + (1 + y_{1})^2 \implies

( m 2 1 ) y 1 2 2 y 1 ( m 2 + 1 ) = 0 y 1 = m 2 + 1 m 2 1 (m^2 - 1)y_{1}^2 - 2y_{1} - (m^2 + 1) = 0 \implies y_{1} = \dfrac{m^2 + 1}{m^2 - 1} \implies

d 1 = A D = 2 m 2 m 2 1 A 1 = m 3 m 2 1 d_{1} = \overline{AD} = \dfrac{2m^2}{m^2 - 1} \implies A_{1} = \dfrac{m^3}{m^2 - 1} .

d 1 = 2 m 2 m 2 1 m 2 + ( d 1 + 1 + y 2 2 ) 2 = m 2 y 2 2 d_{1} = \dfrac{2m^2}{m^2 - 1} \implies m^2 + (d_{1} + 1 + y_{2}^2)^2 = m^2y_{2}^2 \implies

( m 2 1 ) y 2 2 2 ( d 1 + 1 ) y 2 ( m 2 + ( d 1 + 1 ) 2 ) = 0 (m^2 - 1)y_{2}^2 - 2(d_{1} + 1)y_{2} - (m^2 + (d_{1} + 1)^2) = 0 \implies

y 2 = d 1 + 1 + m ( d 1 + 1 ) 2 + m 2 1 m 2 1 y_{2} = \dfrac{d_{1} + 1 + m\sqrt{(d_{1} + 1)^2 + m^2 - 1}}{m^2 - 1}

d 2 = d 1 + 1 + y 2 = m m 2 1 ( ( d 1 + 1 ) m + ( d 1 + 1 ) 2 + m 2 1 ) d_{2} = d_{1} + 1 + y_{2} = \dfrac{m}{m^2 - 1}((d_{1} + 1)m + \sqrt{(d_{1} + 1)^2 + m^2 - 1})

Similarly m 2 + ( d 2 + 1 + y 3 ) 2 = m 2 y 3 2 m^2 + (d_{2} + 1 + y_{3})^2 = m^2y_{3}^2 \implies

( m 2 1 ) y 3 2 2 ( d 2 + 1 ) y 3 ( m 2 + ( d 2 + 1 ) 2 ) = 0 (m^2 - 1)y_{3}^2 - 2(d_{2} + 1)y_{3} - (m^2 + (d_{2} + 1)^2) = 0 \implies

y 3 = d 2 + 1 + m ( d 2 + 1 ) 2 + m 2 1 m 2 1 y_{3} = \dfrac{d_{2} + 1 + m\sqrt{(d_{2} + 1)^2 + m^2 - 1}}{m^2 - 1} \implies

d 3 = d 2 + 1 + y 3 = d 2 + 1 + y 3 = m m 2 1 ( ( d 2 + 1 ) m + ( d 2 + 1 ) 2 + m 2 1 ) d_{3} = d_{2} + 1 + y_{3} = d_{2} + 1 + y_{3} = \dfrac{m}{m^2 - 1}((d_{2} + 1)m + \sqrt{(d_{2} + 1)^2 + m^2 - 1})

In General using A 1 = m 3 m 2 1 \boxed{A_{1} = \dfrac{m^3}{m^2 - 1}}

Let d 1 = 2 m 2 m 2 1 \boxed{d_{1} = \dfrac{2m^2}{m^2 - 1}}

and for n 1 \boxed{n \geq 1} define:

d n + 1 = m m 2 1 ( ( d n + 1 ) m + ( d n + 1 ) 2 + m 2 1 ) \boxed{d_{n + 1} = \dfrac{m}{m^2 - 1}((d_{n} + 1)m + \sqrt{(d_{n} + 1)^2 + m^2 - 1})} and A n + 1 = m 2 d n + 1 \boxed{A_{n + 1} = \dfrac{m}{2}d_{n + 1}}

or if you prefer d n + 1 = m m 2 1 ( ( d n + 1 ) m + d n 2 + 2 d n + m 2 ) d_{n + 1} = \dfrac{m}{m^2 - 1}((d_{n} + 1)m + \sqrt{d_{n}^2 + 2d_{n} + m^2}) and A n + 1 = m 2 d n + 1 A_{n + 1} = \dfrac{m}{2}d_{n + 1}

Using the above recursive formula with m = 2 m = 2 the program below computes A 10 A_{10} using python:

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import math;

d = 8/3;

for n in range(1,10):
       d = 2/3 * (2 * (d + 1) + math.sqrt(pow((d + 1),2) + 3));

print(d);

0 \phantom 0

A 10 = 2473.00493680999 \implies A_{10} = \boxed{2473.00493680999} .

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