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Geometry Level 3

An infinite series of similar triangles converge to a point C . If A E = 16 , E D = 8 \overline{AE} = 16 , \overline{ED} = 8 . Denote x x as the sum of all the vertical segments: A E + B D + \overline{AE} + \overline{BD} + \ldots

Find the value of x x .


The answer is 32.

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2 solutions

Chew-Seong Cheong
Mar 13, 2015

Let A E B = E B D = . . . = θ \angle AEB = \angle EBD = ... = \theta . We note that: A E cos θ = E B AE \cos{\theta} = EB and E B sin θ = E D EB \sin{\theta} = ED .

E B sin θ = E D A E cos θ sin θ = E D 16 cos θ sin θ = 8 2 cos θ sin θ = 1 sin 2 θ = 1 θ = 4 5 \begin{aligned} \Rightarrow EB \sin{\theta} & = ED \\ AE \cos{\theta}\sin{\theta} & = ED \\ 16 \cos{\theta}\sin{\theta} & = 8 \\ 2 \cos{\theta}\sin{\theta} & = 1 \\ \sin{2\theta} & = 1 \\ \Rightarrow \theta & = 45^\circ \end{aligned}

Therefore, the similar triangles are isosceles triangles with the horizontal side same length with vertical side. And the vertical height is half of that of the previous one. Therefore, we have:

x = 16 + 8 + 4 + . . . = 16 ( 1 + 1 2 + 1 4 + . . . ) x=16+8+4+... = 16(1+\frac {1}{2}+\frac {1}{4}+... )

= 16 n = 0 ( 1 2 ) n = 16 1 1 2 = 32 \displaystyle \quad = 16\sum _{n=0} ^\infty {\left( \frac {1} {2} \right)^n } = \dfrac {16}{1-\frac{1}{2}} = 32

FYI. I have updated the question to ask just for x x , and edited your solution accordingly.

Calvin Lin Staff - 6 years, 3 months ago

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Thank you.... :)

Sakanksha Deo - 6 years, 3 months ago

Okay, thanks

Chew-Seong Cheong - 6 years, 3 months ago

By looking the drawing since EB is perpendicular to AC and ED is equal to 1/2 AE, angle BED equal to 45 degrees so DB = ED =1/2EA.

By doing this procedure for every next segment and adding all of them we get a length equal to the first segment AE=16. Since the solution requires we add-up also AE we get 2 AE = 32.

FYI. I have updated the question to ask just for x x , and edited your solution accordingly.

Calvin Lin Staff - 6 years, 3 months ago

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