The oak tree have a liner function and the birch tree have exponential function .
What is linear function?
Linear functions are those whose graph is a straight line. A linear function has the following form. y = f(x) = a + bx. A linear function has one independent variable and one dependent variable. The independent variable is x and the dependent variable is y.
What is exponential function?
An exponential function is a mathematical function of the following form: 
f ( x ) = [tex]a^{x}[/tex]. where x is a variable, and a is a constant called the base of the function. The most commonly encountered exponential-function base is the transcendental number e , which is equal to approximately 2.71828.
According to the question
The number of branches of an oak tree and a birch tree since 1950 are represented by the following tables: 
Number of Oak trees (x)                       Number of branches  f(x)
0                                                                     34
3                                                                     46
6                                                                    59
9                                                                    70
12                                                                   82   
So in this table we can see that there is increment in number of branches but in a pattern approx.
i.e 
f(x) = 34 + 4(x) 
where x will be Number of Oak trees 
therefore it is a Linear function 
 
Number of Birch trees (x)                         Number of branches f(x)
0                                                                       8
10                                                                    33
20                                                                   128
30                                                                   512
40                                                                 2049  
So , in this table is increasing in a very fast pace but in a pattern approx.
i.e 
f(x) = [tex]8*4^{\frac{x}{10} }[/tex] 
Therefore it is an exponential function 
Hence, The oak tree have a liner function and the birch tree have exponential function .
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