SOLUTION: log3 (x-4y+5)=0 and log3 (x-1)-log3 y=1 simaltenously

Algebra ->  Logarithm Solvers, Trainers and Word Problems -> SOLUTION: log3 (x-4y+5)=0 and log3 (x-1)-log3 y=1 simaltenously      Log On


   



Question 1102526: log3 (x-4y+5)=0 and log3 (x-1)-log3 y=1
simaltenously

Answer by Edwin McCravy(20060) About Me  (Show Source):
You can put this solution on YOUR website!

Learn how to change logarithm equations to and from 
equivalent exponential equations:

log%28B%2C%28A%29%29=C and A=B%5EC are equivalent.

Therefore your first equation:

log%283%2C%28x-4y%2B5%29%29=0 is equivalent to

x-4y%2B5=3%5E0 and then
x-4y%2B5=1
x=4y-4

Your second equation:

log%283%2C%28x-1%29%29-log%283%2C%28y%29%29=1

Using a rule of logarithms, becomes

log%283%2C%28%28x-1%29y%5E%22%22%29%29=1, which is equivalent to

%28x-1%29y=3%5E1 and then

%28x-1%29y=3

Substituting 4y-4 for x:

%284y-4-1%29y=3



y+=+%285+%2B-+sqrt%2825%2B48%29%29%2F8+

y+=+%285+%2B-+sqrt%2873%29%29%2F8+

Substituting in x=4y-4

x=4%28%285+%2B-+sqrt%2873%29%29%2F8%29-4

x=%285+%2B-+sqrt%2873%29%29%2F2-4

x=%285+%2B-+sqrt%2873%29%29%2F2-8%2F2

x=%285-8+%2B-+sqrt%2873%29%29%2F2

x=%28-3+%2B-+sqrt%2873%29%29%2F2

Since logs cannot be taken of negative numbers, and
the original second equation contains log%283%2C%28y%29%29 
and log%283%2C%28x-1%29%29, the only solutions are the
positive ones:

x=%28-3+%2B+sqrt%2873%29%29%2F2 and y+=+%285+%2B+sqrt%2873%29%29%2F8+

Edwin