# Julia's question at Yahoo! Answers regarding a linear recurrence

#### MarkFL

Staff member
Here is the question:

A tree farm in the year 2010 has 10,000 Douglas fir trees on its property. Each year thereafter 10% of the fir trees are harvested and 750 new fir saplings are then planted in their place.

a) Write a recursive sequence that gives the current number t"sub"n of fir trees on the farm in the year n, with n=0 corresponding to 2010.

b) Use the recursive formula from part a to find the numbers of fir trees for n=1, 2, 3, and 4. Interpret the values in context.

c) Use a graphing utility to find the number of fir trees as time passes infinitely. Explain your result.

***Please show work and explain how you got the answers because I have no idea what to do! Thanks so much!
Here is a link to the question:

I have posted a link there to this topic so the OP can find my response.

#### MarkFL

Staff member
Hello Julia,

a) Let $T_n$ denote the number of trees present in year $n$. Each year, 10% of the trees are removed (leaving 90%) and 750 trees are added, and at time $n=0$ we have 10000 trees, hence we may state the linear inhomogeneous recurrence:

$$\displaystyle T_{n+1}=0.9T_{n}+750$$ where $$\displaystyle T_0=10000$$

b) Using the result from part a), we may state:

$$\displaystyle T_{1}=0.9T_{0}+750=0.9\cdot10000+750=9750$$ (This is the number of trees in 2011).

$$\displaystyle T_{2}=0.9T_{1}+750=0.9\cdot9750+750=9525$$ (This is the number of trees in 2012).

$$\displaystyle T_{3}=0.9T_{2}+750=0.9\cdot9525+750=9322.5 \approx9323$$ (This is the number of trees in 2013).

$$\displaystyle T_{4}=0.9T_{3}+750=0.9\cdot9322.5+750=9140.25 \approx9140$$ (This is the number of trees in 2014).

c) To find the value of $$\displaystyle \lim_{n\to\infty}T_{n}$$, let's find the closed form.

(1) $$\displaystyle T_{n+1}=0.9T_{n}+750$$

(2) $$\displaystyle T_{n+2}=0.9T_{n+1}+750$$

Subtracting (1) from (2), using symbolic differencing, we obtain the linear homogeneous recurrence:

$$\displaystyle T_{n+2}=1.9T_{n+1}-0.9T_{n}$$

The characteristic roots are:

$$\displaystyle \lambda=0.9,1$$ hence:

$$\displaystyle T_n=k_1+k_2(0.9)^n$$

Using the initial values, we may determine the parameters $k_i$:

$$\displaystyle T_0=k_1+k_2(0.9)^0=k_1+k_2=10000$$

$$\displaystyle T_1=k_1+k_2(0.9)^1=k_1+0.9k_2=9750$$

Solving this system, we find $$\displaystyle k_1=7500,\,k_2=2500$$ and so we have:

$$\displaystyle T_n=7500+2500(0.9)^n$$ and low it is easy to see that:

$$\displaystyle \lim_{n\to\infty}T_{n}=7500$$

Here is a plot of the recurrence:

To Julia and any other guests viewing this topic, I invite and encourage you to post other linear recurrence questions here in our Discrete Mathematics, Set Theory, and Logic forum.

Best Regards,

Mark.

Last edited: