A seven, a three and 2016!?

Algebra Level 4

7 x 3 y = 2016 7^{x} - 3^{y} = 2016

How many real solutions (ordered pairs) satisfy the equation above?

2016 3 2 0 7 None of the offered 1 21

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1 solution

Milan Milanic
Jan 22, 2016

Solution:

This one is more of a trick than a real problem. Since it says " real solutions " and not integers , number of ordered pairs that satisfy the equation is \infty .

Can you explain a little how real solutions will yield infinite solution?

Department 8 - 5 years, 4 months ago

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3 y = m 3^{y} = m ( m m shall be positive integer, to make things easier).

Then 7 x = 2016 + m 7^{x} = 2016 + m

y = log 3 m y = \log _{ 3 }{ m } and x = log 7 ( 2016 + m ) x = \log _{ 7 }{ (2016 + m) }

So if m = 1 m = 1 then: ( x , y ) = ( log 7 2017 , 0 ) (x, y) = (\log _{7 }{ 2017 }, 0) , both values are real.

That applies for m = 2 , 3 , 4 , . . . . . . m = 2, 3, 4, ...... to the infinity.

Milan Milanic - 5 years, 4 months ago

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Wow, that's awesome!!

Department 8 - 5 years, 4 months ago

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