Friday 6 February 2015

`log_3(x) + log_3(x - 2) = 1` Solve for x or b

`log_3(x) + log_3(x - 2) = 1`


The logarithms at the left side have the same base. So express the left side with one logarithm only using the rule `log_b (M) + log_b (N) = log_b(M*N` ).


`log_3(x * (x-2)) = 1`


`log_3 (x^2 - 2x) = 1`


Then, convert this to exponential form.


Take note that if a logarithmic equation is in the form


`y = log_b (x)`


its equivalent exponential equation is


`x...

`log_3(x) + log_3(x - 2) = 1`


The logarithms at the left side have the same base. So express the left side with one logarithm only using the rule `log_b (M) + log_b (N) = log_b(M*N` ).


`log_3(x * (x-2)) = 1`


`log_3 (x^2 - 2x) = 1`


Then, convert this to exponential form.


Take note that if a logarithmic equation is in the form


`y = log_b (x)`


its equivalent exponential equation is


`x = b^y`


So converting


`log_3 (x^2 - 2x) =1`


to exponential equation, it becomes:


`x^2-2x = 3^1`


`x^2 - 2x = 3`


Now the equation is in quadratic form. To solve it, one side should be zero.


`x^2 - 2x - 3 = 0`


Factor the left side.


`(x - 3)(x +1)=0`


Set each factor equal to zero. And isolate the x.


`x - 3 = 0`


`x=3`



`x + 1=0`


`x = -1`


Now that the values of x are known, consider the condition in a logarithm. The argument of a logarithm should always be positive.


In the equation


`log_3(x) + log_3(x - 2)=1`


the arguments are x and x - 2. So the values of these two should all be above zero.


`x gt 0`


`x - 2gt0`


Between the two values of x that we got, it is only x = 3 that satisfy this condition.



Therefore, the solution is `x=2` .

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