Continuity Problem

Calculus Level 4

Let f ( x ) f(x) be a piecewise function such that: f ( x ) = { x 2 32 ( 4 16 ) x + 4 16 1 ( x 1 ) 2 , x 1 k , x = 1 \large f(x) = \begin{cases} \begin{aligned} & \cfrac{x^{2^{32}} - (4^{16})x + 4^{16} - 1}{(x - 1)^2} & , \hspace{1cm} x \ne 1 \\ & k & , \hspace{1cm} x = 1 \end{aligned} \end{cases}

Let k = a b × ( a c 1 ) k = a^b \times (a^c - 1) , where the value of k k make f ( x ) f(x) continuous at 1.

If a a is a prime number and b < c b < c , solve for a + b + c a + b + c .


The answer is 65.

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1 solution

Christian Daang
Feb 20, 2017

Applying the three conditions to make a function continuous at a value a,

1 s t 1^{st} condition: lim x a f ( x ) \large \lim_{x \rightarrow a} f(x) exists.

f ( x ) = x 2 32 ( 4 16 ) x + 4 16 1 ( x 1 ) 2 , a = 1 lim x 1 x 2 32 ( 4 16 ) x + 4 16 1 ( x 1 ) 2 = lim x 1 ( x 2 32 ( 4 16 ) x + 4 16 1 ) lim x 1 ( x 1 ) 2 = lim x 1 ( ( 2 32 ) x 2 32 1 4 16 ) lim x 1 ( 2 ( x 1 ) ) = lim x 1 ( ( 2 32 ) ( 2 32 1 ) x 2 32 2 ) lim x 1 ( 2 ) = lim x 1 ( ( 2 31 ) ( 2 32 1 ) x 2 32 2 ) = ( 2 31 ) ( 2 32 1 ) \large \begin{aligned} f(x) = \cfrac{x^{2^{32}} - (4^{16})x + 4^{16} - 1}{(x - 1)^2} \ , \ a = 1 \\ \implies \lim_{x \rightarrow 1} \cfrac{x^{2^{32}} - (4^{16})x + 4^{16} - 1}{(x - 1)^2} & = \cfrac{\lim_{x \rightarrow 1} \left( x^{2^{32}} - (4^{16})x + 4^{16} - 1 \right) }{\lim_{x \rightarrow 1} (x - 1)^2} \\ & = \color{#D61F06} {\cfrac{\lim_{x \rightarrow 1} \left( \left(2^{32}\right) x^{2^{32} - 1} - 4^{16} \right) }{\lim_{x \rightarrow 1} \Big( 2(x-1) \Big) }} \\ & = \color{#3D99F6} {\cfrac{\lim_{x \rightarrow 1} \left( \left(2^{32}\right) \left(2^{32} - 1\right) x^{2^{32} - 2} \right)}{\lim_{x \rightarrow 1} (2)}} \\ & = \lim_{x \rightarrow 1} \Big( \left(2^{31}\right) \left(2^{32} - 1\right) x^{2^{32} - 2} \Big) \\ & = \left(2^{31}\right) \left(2^{32} - 1\right) \end{aligned}

Note: f ( x ) = x 2 32 ( 4 16 ) x + 4 16 1 ( x 1 ) 2 \large f(x) = \cfrac{x^{2^{32}} - (4^{16})x + 4^{16} - 1}{(x - 1)^2} was used as we are dealing with limits.


2 n d 2^{nd} condition: f ( a ) f(a) exist:

f ( x ) = k , a = 1 f ( 1 ) = k f(x) = k \ , \ a = 1 \\ \implies f(1) = k

Note: f ( x ) = k f(x) = k was used as we are dealing with the function value.


3 r d 3^{rd} condition: lim x a f ( x ) = f ( a ) \large \lim_{x \rightarrow a} f(x) = f(a)

lim x 1 f ( x ) = ( 2 31 ) ( 2 32 1 ) , f ( 1 ) = k k = ( 2 31 ) ( 2 32 1 ) a = 2 , b = 31 , c = 32 a + b + c = 2 + 31 + 32 = 65 \large \lim_{x \rightarrow 1} f(x) = \left(2^{31}\right) \left(2^{32} - 1\right) \ , \ f(1) = k \\ \implies k = \left(2^{31}\right) \left(2^{32} - 1\right) \\ \implies a = 2 \ , \ b = 31 \ , \ c = 32 \\ \therefore \ \boxed{a + b + c = 2 + 31 + 32 = 65}


The red and blue colored ones was obtain by applying L'Hôpital's Rule .

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