Cool functions

Algebra Level 5

Let a function f ( x ) f(x) satisfy :- f ( x ) = a x a x + a \large f(x) = \dfrac{a^{x}}{a^{x} + \sqrt{a}} , ( For a>0) (\text{For a>0)} and r = 0 4 n f ( r 4 n + 1 ) = 1 1 + a + 1974 \large \displaystyle \sum_{r=0}^{4n} f \bigg(\dfrac{r}{4n+1} \bigg) = \dfrac{1}{1+\sqrt{a}} + 1974 ,then find the value of n n


The answer is 987.

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1 solution

Aditya Dhawan
May 9, 2016

N o t e t h a t f ( 1 x ) + f ( x ) = a 1 x a 1 x + a + a x a x + a = a + a 3 2 x + a x + a x + 1 2 a + a 3 2 x + a x + a x + 1 2 = 1 N o w r = 0 4 n f ( r 4 n + 1 ) = 1 1 + a + 1974 r = 1 4 n f ( r 4 n + 1 ) = 1974 { f ( 0 ) = 1 1 + a } r = 1 4 n f ( r 4 n + 1 ) = { [ f ( 1 4 n + 1 ) + f ( 4 n 4 n + 1 ) ] + [ f ( 2 4 n + 1 ) + f ( 4 n 1 4 n + 1 ) ] . . . . . . . . . . . . . . . 2 n t i m e s } = 2 n 2 n = 1974 n = 987 Note\quad that\quad f(1-x)+f(x)=\frac { { a }^{ 1-x } }{ { a }^{ 1-x }+\sqrt { a } } +\frac { { a }^{ x } }{ { a }^{ x }+\sqrt { a } } =\frac { a+{ a }^{ \frac { 3 }{ 2 } -x }+{ a }^{ x }+{ a }^{ x+\frac { 1 }{ 2 } } }{ a+{ a }^{ \frac { 3 }{ 2 } -x }+{ a }^{ x }+a^{ x+\frac { 1 }{ 2 } } } =1\\ \\ Now\quad \sum _{ r=0 }^{ 4n }{ f\left( \frac { r }{ 4n+1 } \right) } =\frac { 1 }{ 1+\sqrt { a } } +1974\quad \equiv \sum _{ r=1 }^{ 4n }{ f\left( \frac { r }{ 4n+1 } \right) } =1974\quad \left\{ \because f(0)=\frac { 1 }{ 1+\sqrt { a } } \right\} \\ \\ \sum _{ r=1 }^{ 4n }{ f\left( \frac { r }{ 4n+1 } \right) } =\left\{ \left[ f\left( \frac { 1 }{ 4n+1 } \right) +f\left( \frac { 4n }{ 4n+1 } \right) \right] +\left[ f\left( \frac { 2 }{ 4n+1 } \right) +f\left( \frac { 4n-1 }{ 4n+1 } \right) \right] ...............2n\quad times \right\} =2n\\ \therefore 2n=1974\Rightarrow \boxed { n=987 } \\

Moderator note:

Good observation of f ( 1 x ) + f ( x ) = 1 f(1-x) + f(x) = 1 .

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