I am Groot!

Algebra Level 2

A tree undergoes a sprouting growth.

Its longest branch is 32 units long, and the pair of branches connected to it are both 1 3 \frac{1}{3} the length of that branch. The same pattern continues with the length consistently shortening by 1 3 \frac{1}{3} in each step. Adjacent branches meet at 12 0 . 120^\circ.

If this tree sprouts an infinite number of branches, what is its total height?


The answer is 42.

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15 solutions

Steven Chase
Jun 20, 2018

The height of the first "Y" portion is:

H 1 = 32 ( 1 + 1 3 s i n ( 3 0 ) ) = 32 ( 1 + 1 3 1 2 ) = 32 7 6 H_1 = 32 \Big (1 + \frac{1}{3} sin(30^\circ) \Big) = 32 \Big (1 + \frac{1}{3} \frac{1}{2} \Big ) = 32 \frac{7}{6}

Then the total height is based on an infinite geometric series with initial term H 1 H_1 and common ratio 1 9 \frac{1}{9} :

H = H 1 1 1 9 = 9 8 H 1 = 9 8 7 6 32 = 42 H = \frac{H_1}{ 1 - \frac{1}{9}} = \frac{9}{8} H_1 = \frac{9}{8} \frac{7}{6} 32 = 42

and is that 32+7/6, or (32*7)/6?

Jaromír Durna - 2 years, 11 months ago

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It's multiplication

Steven Chase - 2 years, 11 months ago

The question was what is the total length, not what was the final height!!!!!

Kermit Rose - 2 years, 11 months ago

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The question asks for total height

Steven Chase - 2 years, 11 months ago

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Hello Steven. Yes, I was too hasty in reading the question. I calculated the total length and was surprised that it was not the right answer. So, when I read the real solution, I was surprised that the answer calculated the height instead of the length. Thank you for your response.

Kermit Rose - 2 years, 11 months ago

We note that the height of the tree is given by:

H = 32 ( 1 + 1 3 cos 6 0 + ( 1 3 ) 2 + ( 1 3 ) 3 cos 6 0 + ( 1 3 ) 4 + ) = 32 ( 1 + 1 3 × 1 2 + ( 1 3 ) 2 + ( 1 3 ) 3 × 1 2 + ( 1 3 ) 4 + ) = 32 ( n = 0 ( 1 3 ) 2 n + 1 2 n = 0 ( 1 3 ) 2 n + 1 ) = 32 ( n = 0 ( 1 9 ) n + 1 6 n = 0 ( 1 9 ) n ) = 32 × 7 6 n = 0 ( 1 9 ) n = 32 × 7 6 × 1 1 1 9 = 32 × 7 6 × 9 8 = 42 \begin{aligned} H & = 32\left(1+\frac 13 \cos 60^\circ + \left(\frac 13\right)^2 + \left(\frac 13\right)^3\cos 60^\circ + \left(\frac 13\right)^4 + \cdots \right) \\ & = 32\left(1+\frac 13 \times \frac 12 + \left(\frac 13\right)^2 + \left(\frac 13\right)^3\times \frac 12 + \left(\frac 13\right)^4 + \cdots \right) \\ & = 32 \left(\sum_{n=0}^\infty \left(\frac 13\right)^{2n} + \frac 12 \sum_{n=0}^\infty \left(\frac 13\right)^{2n+1}\right) \\ & = 32 \left(\sum_{n=0}^\infty \left(\frac 19\right)^n + \frac 16 \sum_{n=0}^\infty \left(\frac 19\right)^n\right) \\ & = \frac {32\times 7}6 \sum_{n=0}^\infty \left(\frac 19\right)^n \\ & = \frac {32\times 7}6 \times \frac 1{1-\frac 19} \\ & = \frac {32\times 7}6 \times \frac 98 \\ & = \boxed{42} \end{aligned}

Noel Lo
Jul 1, 2018

Considering that the diagonal branch is one-third the length of its immediately preceding branch (which is vertical) and that it forms an angle of 12 0 120 ^\circ with this vertical branch, it actually increases the tree's height by a factor of only sin 3 0 × 1 3 = 1 2 × 1 3 = 1 6 \sin 30 ^\circ \times \dfrac{1}{3}=\dfrac{1}{2}\times\dfrac{1}{3}=\dfrac{1}{6} . Now the tree is 7 6 \dfrac{7}{6} its original height with the diagonal branch. After the first diagonal branch, the tree's height grows to 32 × 7 6 = 112 3 32\times\dfrac{7}{6}=\dfrac{112}{3} .

Next we have another vertical branch and the cycle repeats itself. This vertical branch is 1 3 \dfrac{1}{3} the length of the immediately preceding diagonal branch which means it is 1 3 × 1 3 = 1 9 \dfrac{1}{3}\times\dfrac{1}{3}=\dfrac{1}{9} the length of the immediately preceding vertical branch, so with each cycle, the tree's height increases by a factor of 1 9 \dfrac{1}{9} . Employing infinite series, the tree's height will eventually be almost this tall:

112 3 ( 1 + 1 9 + 1 9 2 + . . . ) = 112 3 1 1 9 = 112 3 8 9 = 112 3 9 8 = 14 × 3 = 42 \dfrac{112}{3}(1+\dfrac{1}{9}+\dfrac{1}{9^2}+...)=\dfrac{\frac{112}{3}}{1-\frac{1}{9}}=\dfrac{\frac{112}{3}}{\frac{8}{9}}=\dfrac{112}{3}\dfrac{9}{8}=14\times 3=\boxed{42} .

Binky Mh
Jul 1, 2018

Let's start by working out the relative total height of the vertical lines. As each vertical line is 1 3 × 1 3 = 1 9 \frac{1}{3}\times\frac{1}{3}=\frac{1}{9} of the height of the previous vertical line, we can get the following infinite sum (where v v is the height of the first line):

h v = v + v 9 + v 9 2 + v 9 3 + . . . h_v=v+\frac{v}{9}+\frac{v}{9^2}+\frac{v}{9^3}+ ...

We can solve h v h_v by first dividing both sides by 9 9 to get:

h v 9 = v 9 + v 9 2 + v 9 3 + v 9 4 + . . . \frac{h_v}{9}=\frac{v}{9}+\frac{v}{9^2}+\frac{v}{9^3}+\frac{v}{9^4}+ ...

Which we then substitute into the first series to get:

h v = v + h v 9 h_v=v+\frac{h_v}{9}

Which we can rearrange to:

h v = v × 9 8 h_v=v\times\frac{9}{8}

We now need to include the diagonals. With trigonometry, we can get the height of each diagonal with l e n g t h × cos ( 60 ) = l e n g t h × 1 2 length\times\cos(60) = length \times \frac{1}{2} . This shows that the height of each diagonal is 1 2 × 1 3 = 1 6 \frac{1}{2}\times\frac{1}{3}=\frac{1}{6} of the previous vertical line. By pairing up each diagonal with the previous vertical, we can see that the full height of the tree is 1 + 1 6 = 7 6 1+\frac{1}{6}=\frac{7}{6} of the sum of the heights of all vertical lines ( h v h_v ). From this, we can now work out:

h = h v × 7 6 = ( v × 9 8 ) × 7 6 = v × 21 16 h=h_v\times\frac{7}{6}=(v\times\frac{9}{8})\times\frac{7}{6}=v\times\frac{21}{16}

By plugging in the initial 32 32 , we can get our final answer:

h = 32 × 21 16 = 42 h=32\times\frac{21}{16}=\boxed{42} .

Victor Dumbrava
Jul 2, 2018

Since the angles are equal to 12 0 120^\circ , all are congruent and therefore every second branch will be parallel to the first one, and every other branch will form an angle of 3 0 30^\circ degrees with the horizontal. Hence, we must add the heights of the vertical branches and the halves of the lengths of the remaining ones.

The length of the n th n^{\text{th}} branch (0-indexed) is given by l n = 32 3 n l_n=\frac{32}{3^n} , and we should multiply each l n l_n by either 1 2 \frac{1}{2} or 1 1 , depending on its parity. Hence, we find the function f ( i ) f(i) which maps even values of i i to 1 1 and odd ones to 2 2 :

f ( i ) = ( 1 ) i + 1 + 3 2 f(i)=\frac{(-1)^{i+1}+3}{2}

Now, we can simply plug all those values into a closed-form expression:

H = i = 0 ( 64 3 i ( ( 1 ) i + 1 + 3 ) ) = 42 H=\sum_{i=0}^\infty \left(\frac{64}{3^i\cdot((-1)^{i+1}+3)}\right)=\boxed{42}

...kinda seems to me like f(i) maps odd values to 2 though.

Johannes Rückert - 2 years, 11 months ago

I think you meant to put 1 instead of 3 in your first function.

Youssof Eldeab - 2 years, 10 months ago
Thomas Krüger
Jul 6, 2018

The Ultimate Answer to Life, The Universe and Everything is...42

Kevin Tong
Jul 4, 2018

We classify the branches in two ways: vertical and at a 3 0 o 30^\text{o} angle. Notice, these alternately and consistently form the peak of the tree as we move from one step to another. Furthermore, the branches are multiplied by a factor of 1 3 \frac{1}{3} each time, forming a geometric sequence of factor ( 1 3 ) 2 = 1 9 \left(\frac{1}{3}\right)^2=\frac{1}{9} with these two types of branches. The first vertical branch is of length 32 32 , so its geometric series sums to 32 1 1 9 = 32 8 9 = 4 9 = 36 \frac{32}{1-\frac{1}{9}}=\frac{32}{\frac{8}{9}}=4\cdot 9=36 The first slanted branch has length 32 3 \frac{32}{3} , and forms a 30-60-90 triangle with horizontal and vertical lines, equating to a height of sin 3 0 o 32 3 = 1 2 32 3 = 16 3 \sin 30^\text{o}\cdot \frac{32}{3}=\frac{1}{2}\cdot \frac{32}{3}=\frac{16}{3} . This means its geometric series sums to 16 3 1 1 9 = 16 3 8 9 = 16 3 9 8 = 6 \frac{\frac{16}{3}}{1-\frac{1}{9}}=\frac{\frac{16}{3}}{\frac{8}{9}}=\frac{16}{3}\cdot \frac{9}{8}=6 So the total max height is 36 + 6 = 42 36+6=\boxed{42}

Robert Stewart
Jul 2, 2018

1/3 of 32 is 10.66, which round down to the next whole number of 10. The tree can only reach a height of of the starting plus 1/3 of height of longest branch of 32.

(0.33)32=10.66 10+32=42

Théo Leblanc
Jul 2, 2018

First introduce two sequences:

v = ( v n ) n 0 v=(v_n)_{n \geq 0} gives the length of the n t h n^{th} branch ( n n starts at 0) and u = ( u n ) n 0 u=(u_n)_{n \geq 0} the gain in altitude of the n t h n^{th} branch. (We consider only one path of branch, they are all equivalents).

It is easy to see that: n 0 , v n = 1 3 n × 32 \forall n \geq 0, v_n=\frac{1}{3^n} \times 32 And: n 0 , u 2 n = v 2 n , u 2 n + 1 = 1 2 v 2 n + 1 \forall n \geq 0, u_{2n}=v_{2n}, u_{2n+1}=\frac{1}{2}v_{2n+1} because cos 12 0 = 1 2 |\cos 120^\circ|=\frac{1}{2}

We search this value (if it converges) n = 0 u n \displaystyle \sum_{n=0}^{\infty} u_n

Note that u u and v v are positive sequences, n 0 , v n u n \forall n \geq 0, v_n \geq u_n and n 0 v n \displaystyle \sum_{n \geq 0}^{} v_n converges, it is a geometric series with r = 1 3 , r < 1 r=\frac{1}{3}, |r|<1 , thus n 0 u n \displaystyle \sum_{n \geq 0}^{} u_n converges let β \beta its value ( β = n = 0 u n ) \beta = \displaystyle \sum_{n=0}^{\infty} u_n) .

β = n = 0 u 2 n + n = 0 u 2 n + 1 = n = 0 32 1 3 2 n + n = 0 16 1 3 2 n + 1 \beta= \displaystyle \sum_{n=0}^{\infty} u_{2n} +\displaystyle \sum_{n=0}^{\infty} u_{2n+1} = \displaystyle \sum_{n=0}^{\infty} 32\frac{1}{3^{2n}} + \displaystyle \sum_{n=0}^{\infty} 16\frac{1}{3^{2n+1}}

β = 32 n = 0 1 9 n + 16 3 n = 0 1 9 n \beta = 32 {\displaystyle \sum_{n=0}^{\infty} \frac{1}{9^n}}+\frac{16}{3}{\displaystyle \sum_{n=0}^{\infty} \frac{1}{9^n}}

1 9 < 1 |\frac{1}{9}|<1 so:

β = 1 1 1 9 ( 32 + 16 3 ) = 42 \beta = \frac{1}{1-\frac{1}{9}}(32+\frac{16}{3})=42

Therefore, the answer is 42 \boxed{42}

Donglin Loo
Jun 20, 2018

Similar to @Steven Chase 's solution

Relevant wiki: Sum of a Geometric Progression with Infinite Terms

Let the length of segment at stage n n be l n l_{n}

l 0 = 32 m l_{0}=32m

We know that l n + 1 = 1 3 l n l_{n+1}=\cfrac{1}{3}l_{n}

l n + 1 = ( 1 3 ) n + 1 l 0 l n = ( 1 3 ) n l 0 l_{n+1}=(\cfrac{1}{3})^{n+1}l_{0}\Rightarrow l_{n}=(\cfrac{1}{3})^{n}l_{0}

Let the height at stage n n be h n h_{n}

h 0 = l 0 = 32 m h_{0}=l_{0}=32m

l 1 = 1 3 l 0 = 1 3 h 0 l_{1}=\cfrac{1}{3}l_{0}=\cfrac{1}{3}h_{0}

h 1 = h 0 + l 1 s i n 3 0 = h 0 + 1 3 l 0 s i n 3 0 = h 0 + 1 3 h 0 s i n 3 0 = h 0 + 1 3 h 0 1 2 = h 0 ( 1 + 1 6 ) h_{1}=h_{0}+l_{1}sin30^\circ=h_{0}+\cfrac{1}{3}l_{0}sin30^\circ=h_{0}+\cfrac{1}{3}h_{0}sin30^\circ=h_{0}+\cfrac{1}{3}h_{0}\cdot \cfrac{1}{2}=h_{0}(1+\cfrac{1}{6})

h 2 = h 1 + l 2 = h 1 + ( 1 3 ) 2 l 0 = h 1 + ( 1 3 ) 2 h 0 = h 0 ( 1 + 1 6 + 1 9 ) h_{2}=h_{1}+l_{2}=h_{1}+(\cfrac{1}{3})^{2}l_{0}=h_{1}+(\cfrac{1}{3})^{2}h_{0}=h_{0}(1+\cfrac{1}{6}+\cfrac{1}{9})

h 3 = h 2 + l 3 s i n 3 0 = h 2 + ( 1 3 ) 3 l 0 1 2 = h 2 + ( 1 3 ) 3 h 0 1 2 = h 0 ( 1 + 1 6 + 1 9 + ( 1 3 ) 3 1 2 ) = h 0 ( 1 + 1 6 + 1 9 + 1 9 1 6 ) = h 0 ( 1 + 1 6 ) ( 1 + 1 9 ) h_{3}=h_{2}+l_{3}sin30^\circ=h_{2}+(\cfrac{1}{3})^{3}l_{0}\cdot \cfrac{1}{2}=h_{2}+(\cfrac{1}{3})^{3}h_{0}\cdot \cfrac{1}{2}=h_{0}(1+\cfrac{1}{6}+\cfrac{1}{9}+(\cfrac{1}{3})^{3}\cdot \cfrac{1}{2})=h_{0}(1+\cfrac{1}{6}+\cfrac{1}{9}+\cfrac{1}{9}\cdot\cfrac{1}{6})=h_{0}(1+\cfrac{1}{6})(1+\cfrac{1}{9})

lim n h n = h 0 ( 1 + 1 6 ) ( 1 + 1 9 + ( 1 9 ) 2 + . . . ) = h 0 7 6 1 1 1 9 = h 0 7 6 9 8 = 32 7 6 9 8 = 42 m \therefore \lim_{n\to \infty} h_{n}=h_{0}(1+\cfrac{1}{6})(1+\cfrac{1}{9}+(\cfrac{1}{9})^{2}+...)=h_{0}\cdot \cfrac{7}{6}\cdot \cfrac{1}{1-\cfrac{1}{9}}=h_{0}\cdot \cfrac{7}{6}\cdot \cfrac{9}{8}=32\cdot \cfrac{7}{6}\cdot \cfrac{9}{8}=\boxed{42}m

Each branch is 1/3 its previous, this means that the total length of one branch-line (the only one which moves constantly up - there are two, but identical) is a + a/3 + a/9 + a/27 + ... or a + a/3^1 + a/3^2 + a/3^3 +..., where a is the size of the starting branch. But each second branch is tilted in a way that 3 new branches are formed with an angle of 120deg to each other. This means that each second branch is a hypotenuse in a right-triangle whose height is x*sin(30deg) where x is the hypotenuse, as 120deg (angle diff) - 90deg (angle from vertical branch to horizontal line at 90deg to height) = 30deg. sin(30deg) = 1/2, so each second term of our infinite series has to be multiplied by 1/2.

So we get the series a + a/2 3 + a/9 + a/2 27 + ..., which can be "split" into the two following series:

a/2 + a/2 3 + a/2 9 + a/2 27 + ... = 1/2(a + a/3 + a/9 + a/27 + ...) and a/2 + a/2 9 + a/2 27 + a/2 243 + ... = 1/2(a + a/9 + a/27 + a/243 + ...)

These series are simple geometric series, each multiplied by 1/2. Solving each of the above geometric series yields 3a/2 and 9a/8, respectively. When you multiply by 1/2, you get 3a/4 and 9a/16. Adding these two sums (in order to get the sum of the original series back) yields 21a/16. As a is 32 (given by the problem), 21*32/16 yields 42.

Christian Helms
Jul 4, 2018

First note that the "Y" portion of the tree repeats itself infinitely. The infinite sum of the height of the smaller getting "Y" portions converges which yields the height of the entire tree. Counting from the base branch with n = 0 n=0 all vertical parts of the "Y" portions are "even". Therefore their length can be described by ( 1 3 ) 2 n × 32 (\frac {1}{3})^{2n} \times 32 or ( 1 9 ) n × 32 (\frac {1}{9})^n \times 32 . The length of the "odd" parts (the branches which are not vertical) is satisfied by the term ( 1 3 ) 2 n + 1 × 32 × cos 6 0 (\frac{1}{3})^{2n+1} \times 32 \times \cos 60^\circ .

Hence, the height of the tree is given by the following term:

n = 0 ( 1 9 ) n × 32 + ( 1 3 ) 2 n + 1 × 32 × cos 6 0 \displaystyle \sum_{n=0}^\infty (\frac {1}{9})^n\times 32 +(\frac{1}{3})^{2n+1} \times 32 \times \cos 60^\circ

= 32 n = 0 ( 1 9 ) n + ( 1 3 ) 2 n + 1 × cos 6 0 = 32 \displaystyle \sum_{n=0}^\infty (\frac {1}{9})^n +(\frac{1}{3})^{2n+1} \times \cos 60^\circ

= 32 n = 0 ( 1 9 ) n + ( 1 3 ) 2 n + 1 × 1 2 = 32 \displaystyle \sum_{n=0}^\infty (\frac {1}{9})^n +(\frac{1}{3})^{2n+1} \times \frac{1}{2}

= 32 n = 0 ( 1 9 ) n + 1 3 × ( 1 3 ) 2 n × 1 2 = 32 \displaystyle \sum_{n=0}^\infty (\frac {1}{9})^n +\frac{1}{3} \times (\frac{1}{3})^{2n} \times \frac{1}{2}

= 32 n = 0 ( 1 9 ) n + 1 6 × ( 1 3 ) 2 n = 32 \displaystyle \sum_{n=0}^\infty (\frac {1}{9})^n +\frac{1}{6} \times (\frac{1}{3})^{2n}

= 32 n = 0 ( 1 9 ) n + 1 6 × ( 1 3 × 1 3 ) n = 32 \displaystyle \sum_{n=0}^\infty (\frac {1}{9})^n +\frac{1}{6} \times (\frac{1}{3} \times \frac{1}{3})^{n}

= 32 n = 0 6 6 × ( 1 9 ) n + 1 6 × ( 1 9 ) n = 32 \displaystyle \sum_{n=0}^\infty \frac{6}{6} \times(\frac {1}{9})^n +\frac{1}{6} \times (\frac{1}{9})^{n}

= 32 × 7 6 n = 0 ( 1 9 ) n = 32 × 7 6 × 1 1 1 9 = 32 \times \frac{7}{6}\displaystyle \sum_{n=0}^\infty (\frac {1}{9})^n = \frac{32 \times 7}{6} \times \frac{1}{1-\frac{1}{9}}

= 32 × 7 6 × 9 8 = 32 × 7 × 9 6 × 8 = 32 × 63 48 = 2 × 63 3 = 126 3 = 42 {= 32 \times 7}{6} \times \frac{9}{8} = \frac{32 \times 7 \times 9}{6 \times 8} = \frac{32 \times 63}{48} = \frac{2 \times 63}{3} = \frac{126}{3} = \boxed{42}

The solution might seem too long at first glance, but this is solely due to the extremely detailed conversion!

Christian Helms - 2 years, 11 months ago
Martin Kockel
Jul 4, 2018

This is a very diffiult problem on its own. But considering that the answer to the meaning of life, the universe and everything is always 42, the answer must be 42.

Matt Miguel
Jul 4, 2018

The answer to life the universe and everything is 42. This question is part of everything so the answer is 42. QED

Indexing terms starting at zero, the even terms will have vertical branches and odd terms will be off vertical by 30 degrees, so the effective height will be sin(30 deg) = 1/2.

Even terms are sum(32/3^(2k) for k in Natural Numbers) Odd terms are 1/2*sum(32/3^(2k+1) for k in Natural Numbers)

Simplifying, even=32*geom(1/9) where geom(x) = sum(x^k for k in Natural Numbers) = 1/(1-x) if abs(x)<1.

Odd = (32/2)(1/3) geom(1/9) =16/3 geom(1/9)

geom(1/9)=1/(1-1/9)=9/8

So total is (32)*(9/8) + (16/3)(9/8) = 36 + 6 = 42

So the vertical branches are responsible for a total height of 36, most of which comes from the initial branch of size 32. The cockeyed branches are responsible for a total height of 6.

Ankush Menat
Jul 3, 2018

Solved using spreadsheet program. A indicates angled branches, B is vertical branches. Answer is 42, of-course.

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