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Q.

If f(9) = 9 , f′(9) = 4 , then find the value of given limit: f(x)-3x 


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a

4

b

14

c

12

d

12 

answer is A.

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Detailed Solution

According to the problem, we are given that f(9)=9 , f′(9)=4 and we need to find the value of the limit  f(x)-3x  .
Let us assume L =  f(x)-3x .
L = f(9)-39 
L = f(9)-33-3  L = 3-30
L = 00, which is an indeterminate form.
So, we can make use of the L-Hospital rule.
Let us recall the L-Hospital rule. We know that if g(x)h(x) . comes out to be indeterminate form (00, ,....), then the given limit can be found as g(x)h(x) =g'xh'(x)=g"(x)h"(x)  
Now, let us apply the L-Hospital rule to the given limit.
So, we have
 L = ddx(fx-3)ddx(x-3)  .
 L = ddxf(x)-ddx3ddx(x)-ddx(3)  .
We know that ddx(x )=12x and dcdx = 0, where c is a constant.
 L = 12f(x)ddx fx-012x-0 
. L = f'(x)2f(x)12x 
L = f'(9)2f(9)129 
. L = 4913
L = 4313
L = 4
So, we have found the value of the given limit as 4.
Therefore, The correct option for the given problem is 1.
 
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