Product of exponents

Algebra Level 2

If x x and y y are positive integers such that 6 x 1 4 y = 48 4 2 y 6^x14^y = 48\cdot42^y , what is ( x , y ) (x,y) ?

( 3 , 5 ) (3,5) ( 3 , 4 ) (3,4) ( 4 , 4 ) (4,4) ( 4 , 3 ) (4,3) ( 5 , 4 ) (5,4)

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2 solutions

Zee Ell
Apr 15, 2017

6 x 1 4 y = 48 × 4 2 y 6^x 14^y = 48 × 42^y

6 x = 48 × 3 y 6^x = 48 × 3^y

2 x 3 x = 2 4 × 3 × 3 y 2^x 3^x = 2^4 × 3 × 3^y

2 x 4 = 3 y + 1 x 2^{x - 4} = 3^{y + 1 - x}

Since both exponents are integers and 2 and 3 are coprimes, therefore the equation above can only stand, when both exponents are zero.

This gives us:

x 4 = 0 x = 4 x - 4 = 0 \Rightarrow x = 4

and

y + 1 x = 0 y + 1 4 = 0 y = 3 y + 1- x = 0 \Rightarrow y + 1 - 4 = 0 \Rightarrow y = 3

Hence, our answer should be:

( 4 , 3 ) \boxed { (4, 3) }

Frodo Baggins
Apr 14, 2017

Factoring the exponents, we get 2 x 3 x 2 y 7 y = 2 4 3 1 2 y 3 y 7 y 2^x\cdot3^x\cdot2^y\cdot7^y = 2^4\cdot3^1\cdot2^y\cdot3^y\cdot7^y . Dividing by 7 y 7^y and combining equal-base exponents we get 2 x + y 3 x = 2 4 + y 3 1 + y 2^{x+y}\cdot3^x = 2^{4+y}\cdot3^{1+y} . Looking at powers of 2, x + y = 4 + y x+y = 4+y , so x = 4 x = 4 . Looking at powers of 3, x = 1 + y x = 1+y , so y = 3 y=3 . Our ordered pair is ( 4 , 3 ) (4,3) .

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