HOW TO ESTIMATE LIMITS FROM GRAPHS

Required Condition for Existence of Limit of Function

lim x->x₀ f(x)  =  L exists if the following hold :

(i) lim x->x₀+ f(x) exists, 

(ii)  lim x->x₀- f(x) exists, and

(iii)  lim x->x₀+ f(x)  =   lim x->x₀- f(x)  =  L

How can we say the function is not exists by looking at its graph ?

When we get different values as x₀ approaches from left and from right, we may say that the function does not exists.

Question 1 :

Use the graph to find the limits (if it exists). If the limit does not exist, explain why?

lim _x->2 f(x)  

Where f(x)  =  4 - x    x ≠ 2

                       0          x = 2

Solution :

To find the value of left hand limit and right hand limit for x -> 2, we have to use the function f(x)  =  4 - x. It is enough to check if we get equal values for left hand and right hand limit.

f(x)  =  0 at x = 2

lim _x->2- f(x)  =  lim _x->2+ f(x) 

Hence the required limit 2.

Question 2 :

Use the graph to find the limits (if it exists). If the limit does not exist, explain why?

lim _x->1 f(x) 

Where f(x)  =  x² + 2      x ≠ 1

                    =    1            x = 1

Solution :

To find the value of left hand limit and right hand limit for x -> 1, we have to use the function f(x)  =  (x² + 2). It is enough to check if we get equal values for left hand and right hand limit.

lim _x->1^- f(x)  =  lim _x->1+ f(x)

Hence the required limit is 3.

Question 3 :

Use the graph to find the limits (if it exists). If the limit does not exist, explain why?

lim _x->3 1/(x- 3)

Solution :

From the graph given above, we get different values for left hand limit and right hand limit.

The function does not exist at x - >3.

Question 4 :

Use the graph to find the limits (if it exists). If the limit does not exist, explain why?

lim _x->5 |x - 5|/(x - 5)

Solution :

From the graph given above, we get different values for left hand limit and right hand limit.

The function does not exist at x - >5.

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