Find the Shaded Region

Geometry Level pending

In the above diagram, B C A E B D \angle{BCA} \cong \angle{EBD} and A E = D C = x , B C = 6 x 1 \overline{AE} = \overline{DC} = x, \overline{BC} = 6x - 1 and E D = 2 x \overline{ED} = 2x .

If the pink area A p = α β α γ ( ω + γ γ λ ) A_{p} = \dfrac{\sqrt{\alpha}}{\beta \alpha^{\gamma}}(\omega + \gamma^{\gamma}\sqrt{\lambda}) , where α , β , γ , ω \alpha, \beta, \gamma, \omega and λ \lambda are coprime positive integers, find α + β + γ + ω + λ \alpha + \beta + \gamma + \omega + \lambda .


The answer is 50.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

3 solutions

Rocco Dalto
Feb 2, 2021

In E B D \triangle{EBD} we have:

λ 1 + 180 λ 3 + λ 2 = 180 λ 3 = λ 1 + λ 2 \lambda_{1} + 180 - \lambda_{3} + \lambda_{2} = 180 \implies \lambda_{3} = \lambda_{1} + \lambda_{2}

and tan ( λ 3 ) = h x , tan ( λ 2 ) = h 3 x \tan(\lambda_{3}) = \dfrac{h}{x}, \tan(\lambda_{2}) = \dfrac{h}{3x} and tan ( λ 1 ) = h 4 x \tan(\lambda_{1}) = \dfrac{h}{4x} \implies

tan ( λ 1 ) = 1 4 tan ( λ 3 ) = 1 4 tan ( λ 1 + λ 2 ) \tan(\lambda_{1}) = \dfrac{1}{4}\tan(\lambda_{3}) = \dfrac{1}{4}\tan(\lambda_{1} + \lambda_{2})

and

tan ( λ 2 ) = 1 3 tan ( λ 1 + λ 2 ) 4 tan ( λ 1 ) = 3 tan ( λ 2 ) \tan(\lambda_{2}) = \dfrac{1}{3}\tan(\lambda_{1} + \lambda_{2}) \implies 4\tan(\lambda_{1}) = 3\tan(\lambda_{2}) \implies

tan ( λ 2 ) = 4 3 tan ( λ 1 ) \tan(\lambda_{2}) = \dfrac{4}{3}\tan(\lambda_{1}) and 4 tan ( λ 1 ) = tan ( λ 1 + λ 2 ) = 7 tan ( λ 1 ) 3 4 tan 2 ( λ 1 ) 4\tan(\lambda_{1}) = \tan(\lambda_{1} + \lambda_{2}) = \dfrac{7\tan(\lambda_{1})}{3 - 4\tan^2(\lambda_{1})} \implies

tan ( λ 1 ) ( 16 tan 2 ( λ 1 ) 5 ) = 0 \tan(\lambda_{1})(16\tan^2(\lambda_{1}) - 5) = 0 and tan ( λ 1 ) 0 tan ( λ 1 ) = 5 4 \tan(\lambda_{1}) \neq 0 \implies \tan(\lambda_{1}) = \dfrac{\sqrt{5}}{4} \implies

h = 4 x tan ( λ 1 ) = 5 x 21 x 2 = 36 x 2 12 x + 1 h = 4x\tan(\lambda_{1}) = \sqrt{5}x \implies 21x^2 = 36x^2 - 12 x + 1 in right A B C \triangle{ABC}

15 x 2 12 x + 1 = 0 x = 6 ± 21 15 \implies 15x^2 - 12x + 1 = 0 \implies x = \dfrac{6 \pm \sqrt{21}}{15} and x = 6 21 15 6 x 1 < 0 x = \dfrac{6 - \sqrt{21}}{15} \implies 6x - 1 < 0

\therefore we choose x = 6 + 21 15 x = \dfrac{6 + \sqrt{21}}{15} h = 5 ( 6 + 21 15 ) \implies h = \sqrt{5}(\dfrac{6 + \sqrt{21}}{15}) \implies

A p = x h = 5 ( 6 + 21 15 ) 2 = 5 3 5 2 ( 19 + 2 2 21 ) = A_{p} = xh =\sqrt{5}(\dfrac{6 + \sqrt{21}}{15})^2 = \dfrac{\sqrt{5}}{3 * 5^2}(19 + 2^2\sqrt{21}) =

α β α γ ( ω + γ γ λ ) α + β + γ + ω + λ = 50 \dfrac{\sqrt{\alpha}}{\beta * \alpha^{\gamma}}(\omega + \gamma^{\gamma}\sqrt{\lambda}) \implies \alpha + \beta + \gamma + \omega + \lambda = \boxed{50} .

Saya Suka
Feb 4, 2021

AB² = BC² - AC²
= (6x - 1)² - (4x)²
= 20x² - 12x + 1

In the same way using Pythagoras, we get
BE² = 21x² - 12x + 1
(and
BD² = 29x² - 12x + 1
though not necessary)

Triangle BED is similar (not congruent, right?) to triangle CEB.
ED / BE = EB / CE
(ED)(CE) = BE²
6x² = 21x² - 12x + 1
15x² - 12x + 1 = 0
x = (2 ± √(7/3)) / 5
Since 6x - 1 > 0 ==> x > 1/6,
x = (2 + √(7/3)) / 5

Pink area
= x√(20x² - 12x + 1)
= √5 (19 + 2²√21) / ((3)(5²))
by substitution.

Answer
= 5 + 3 + 2 + 19 + 21
= 50

Let B C A = E B D = θ \angle BCA = \angle EBD = \theta , A B E = α \angle ABE = \alpha , and A B = h AB = h . By Pythagorean theorem ,

A B 2 = B C 2 C A 2 h 2 = ( 6 x 1 ) 2 + ( 4 x ) 2 = 20 x 2 12 x + 1 \begin{aligned} AB^2 & = BC^2 - CA^2 \\ h^2 & = (6x-1)^2 + (4x)^2 = 20x^2 - 12x + 1 \end{aligned}

Consider tan A B D = tan ( α + θ ) \tan \angle ABD = \tan (\alpha + \theta) ,

tan ( α + θ ) = tan α + tan θ 1 tan α tan θ Note that tan α = A E A B = x h 3 x h = x h + h 4 x 1 x h h 4 x and tan θ = A B C A = h 4 x 3 x h = 4 x 2 + h 2 3 x h 9 x 2 = 4 x 2 + h 2 5 x 2 = h 2 Note that h 2 = 20 x 2 12 x + 1 15 x 2 12 x + 1 = 0 x = 6 + 21 15 Since 6 x 1 > 0 A p = 2 x h 2 = x h = 5 x 2 and h 2 = 5 x 2 = 5 ( 6 + 21 15 ) 2 = 5 3 5 2 ( 19 ± 2 2 21 ) \begin{aligned} \tan (\alpha + \theta) & = \frac {\tan \alpha + \tan \theta}{1-\tan \alpha \tan \theta} & \small \blue{\text{Note that }\tan \alpha = \frac {AE}{AB} = \frac xh} \\ \frac {3x}h & = \frac {\frac xh + \frac h{4x}}{1-\frac xh \cdot \frac h{4x}} & \small \blue{\text{and }\tan \theta = \frac {AB}{CA} = \frac h{4x}} \\ \frac {3x}h & = \frac {4x^2 + h^2}{3xh} \\ 9x^2 & = 4x^2 + h^2 \\ 5x^2 & = \blue{h^2} & \small \blue{\text{Note that }h^2 = 20x^2 -12x + 1} \\ 15 x^2 - 12x + 1 & = 0 \\ \implies x & = \frac {6 + \sqrt{21}}{15} & \small \blue{\text{Since }6x-1>0} \\ \implies A_p & = \frac {2x \cdot h}2 = xh = \sqrt 5x^2 & \small \blue{\text{and }h^2 = 5x^2} \\ & = \sqrt 5 \left(\frac {6 + \sqrt{21}}{15}\right)^2 \\ & = \frac {\sqrt 5}{3\cdot 5^2}\left(19 \pm 2^2\sqrt{21} \right) \end{aligned}

Therefore the required answer is 5 + 3 + 2 + 19 + 21 = 50 5+3+2+19+21 = \boxed{50}

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...