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Algebra Level 3

How many unique real solutions does the following equation have?

( x 2 7 x + 11 ) x 2 19 x + 48 = 1 \large (x^2-7x+11)^{x^2-19x+48}=1

3 6 1 2 4 5

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3 solutions

Parth Sankhe
Jan 5, 2019

Either ( a n y t h i n g ) 0 (anything) ^0 , or ( 1 ) a n y t h i n g (1)^{anything} , or ( 1 ) e v e n i n t e g e r (-1)^{even integer}

Solve the quadratics for these values, and you'll get the answers as 2 , 3 , 4 , 5 , 16 2,3,4,5,16

Let f ( x ) = g ( x ) h ( x ) f(x) = g(x)^{h(x)} , where g ( x ) = x 2 7 x + 11 g(x) = x^2 - 7x+11 and h ( x ) = x 2 19 x + 48 h(x) = x^2-19x+48 . We note that f ( x ) = 1 f(x) = 1 , when

{ f ( x ) = 1 h ( x ) i.e. g ( x ) = 1 for all real h ( x ) f ( x ) = ( 1 ) 2 n i.e. g ( x ) = 1 and h ( x ) = even integer f ( x ) = g ( x ) 0 i.e. h ( x ) = 0 for all real g ( x ) except 0 \begin{cases} f(x) = 1^{h(x)} & \text{i.e. }g(x) = 1 \text{ for all real } h(x) \\ f(x) = (-1)^{2n} & \text{i.e. }g(x) = -1 \text{ and }h(x) = \text{ even integer} \\ f(x) = g(x)^0 & \text{i.e. }h(x) = 0 \text{ for all real } g(x) \text{ except 0} \end{cases}

Case f ( x ) = 1 h ( x ) f(x) = 1^{h(x)} :

g ( x ) = 1 x 2 7 x + 11 = 1 x 2 7 x + 10 = 0 ( x 2 ) ( x 5 ) = 0 \begin{aligned} \implies g(x) & = 1 \\ x^2-7x+11 & = 1 \\ x^2-7x+10 & = 0 \\ (x-2)(x-5) & = 0 \end{aligned}

x = { 2 h ( 2 ) = 14 f ( 2 ) = 1 5 h ( 5 ) = 22 f ( 5 ) = 1 \implies x = \begin{cases} 2 & \implies h(2) = 14 & \implies f(2) = 1 \\ 5 & \implies h(5) = -22 & \implies f(5) = 1 \end{cases}

g ( x ) = 1 x 2 7 x + 11 = 1 x 2 7 x + 12 = 0 ( x 3 ) ( x 4 ) = 0 \begin{aligned} \implies g(x) & = -1 \\ x^2-7x+11 & = -1 \\ x^2-7x+12 & = 0 \\ (x-3)(x-4) & = 0 \end{aligned}

x = { 3 h ( 3 ) = 0 f ( 3 ) = 1 4 h ( 4 ) = 12 f ( 4 ) = 1 \implies x = \begin{cases} 3 & \implies h(3) = 0 & \implies f(3) = 1 \\ 4 & \implies h(4) = -12 & \implies f(4) = 1 \end{cases}

Case f ( x ) = g ( x ) 0 f(x) = g(x)^0 :

h ( x ) = 0 x 2 19 x + 48 = 0 ( x 3 ) ( x 16 ) = 0 \begin{aligned} \implies h(x) & = 0 \\ x^2-19x+48 & = 0 \\ (x-3)(x-16) & = 0 \end{aligned}

x = { 3 g ( 3 ) = 1 f ( 3 ) = 1 16 g ( 16 ) = 155 f ( 16 ) = 1 \implies x = \begin{cases} 3 & \implies g(3) = -1 & \implies f(3) = 1 \\ 16 & \implies g(16) = 155 & \implies f(16) = 1 \end{cases}

Therefore, there are 5 \boxed 5 unique solutions of x = 2 , 3 , 4 , 5 , 16 x=2,3,4,5,16 .

Jordan Cahn
Jan 7, 2019

As noted by @Parth Sankhe , if a b = 1 a^b=1 for real a a and b b , then one of three cases applies:

  • a = 1 a=1 -- then x 2 7 x + 11 = 1 ( x 2 ) ( x 5 ) = 0 x^2-7x+11=1\implies (x-2)(x-5)=0 and x = 2 x=2 or x = 5 x=5 .
  • b = 0 b=0 -- then x 2 19 x + 48 = 0 ( x 3 ) ( x 16 ) = 0 x^2 -19x + 48 = 0 \implies (x-3)(x-16)=0 and x = 3 x=3 or x = 3 x=3 (note that neither of these are roots of x 2 7 x + 11 x^2 - 7x + 11 )
  • a = 1 a=-1 and b b is even -- then x 2 7 x + 11 = 1 ( x 3 ) ( x 4 ) = 0 x^2-7x+11=-1 \implies (x-3)(x-4) = 0 and x = 3 x=3 or x = 4 x=4 . We already know that x = 3 x=3 is a solution. Substituting x = 4 x=4 yields ( 1 ) 68 (-1)^{68} a solution.

Thus, there are 5 \boxed{5} solutions.

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