Thursday, September 11, 2014

Solve for x :- log[base 4](x-1) = log[base 2](x-3)

We'll change the base 4 of the logarithm into the base 2:


 log[base 4](x-1) = log[base 2](x-3)


 log 2 (x-1) = log 4 (x-1)*log 2 (4)


But log 2 (4) = log 2 (2^2)


We'll apply the power property of logarithms and we'll get:


 log 2 (2^2) = 2*log 2 (2)


But log 2 (2) = 1


 log 2 (2^2) = 2


log[base 4](x-1) = log[base 2](x-1)/2


The equation will become:


{log[base 2](x-1)}/2 = log[base 2](x-3)


log[base 2](x-1) = 2*log[base 2](x-3)


log[base 2](x-1) = log[base 2][(x-3)^2]


We'll apply one to one property:


x-1 = (x-3)^2


We'll expand the square:


x-1 = x^2 - 6x + 9


We'llmove all terms to one side:


x^2 - 7x + 10 = 0


We'll apply the quadratic formula:


x1 = [7+sqrt(49-40)]/2


x1 = (7+3)/2


x1 = 5


x2 = (7-3)/2


x2 = 2


Now, we'll check if the found solutions are convenient and respect the constraints of existence of logarithms above:


The constraints of existence of logarithms are:


x-1>0


x>1


x-3>0


x>3


So, all solutions have to be more than the value 3.


Because the second solution, x2=2<3, is not acceptable, so the only solution of the equation is x=5.

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