SOLUTION: I am really struggling to find the answer to this logarithmic equation that we have been set: 6*(3^x)+4*(3^-x)-14=0 Any help would be appreciated, Many thanks, Trevor

Algebra ->  Logarithm Solvers, Trainers and Word Problems -> SOLUTION: I am really struggling to find the answer to this logarithmic equation that we have been set: 6*(3^x)+4*(3^-x)-14=0 Any help would be appreciated, Many thanks, Trevor      Log On


   



Question 233595: I am really struggling to find the answer to this logarithmic equation that we have been set:
6*(3^x)+4*(3^-x)-14=0
Any help would be appreciated,
Many thanks, Trevor

Answer by Edwin McCravy(20064) About Me  (Show Source):
You can put this solution on YOUR website!
I am really struggling to find the answer to this logarithmic equation that we have been set:
+6%2A%283%5Ex%29%2B4%2A%283%5E%28-x%29%29-14=0

Divide through by 2 since all coefficients are even:

+6%2A%283%5Ex%29%2F2%2B4%2A%283%5E%28-x%29%29%2F2-14%2F2=0%2F2

3%2A%283%5Ex%29%2B2%2A%283%5E%28-x%29%29-7=0

Write 3%5E%28-x%29 as 1%2F%283%5Ex%29

3%2A%283%5Ex%29%2B2%2A%281%2F3%5Ex%29-7=0

Multiply through by 3%5Ex to clear of fractions:

3%2A%283%5Ex%29%283%5Ex%29%2B2%2A%281%2F3%5Ex%29%283%5Ex%29-7%283%5Ex%29=0%283%5Ex%29

Simplify:


3%2A%283%5Ex%29%5E2%2B2%2A%281%2Fcross%283%5Ex%29%29%28cross%283%5Ex%29%29-7%283%5Ex%29=0


3%2A%283%5Ex%29%5E2%2B2-7%283%5Ex%29=0

Replace 3%5Ex by u

3u%5E2%2B2-7u=0

Swap the second and third term on the left to get
in descending order or powers of u

3u%5E2-7u%2B2=0

Factor the left side:

%28u-2%29%283u-1%29=0

Set each factor = 0

u-2=0
u=2

3u-1=0
3u=1
u=1%2F3

For each solution for u, replace u by 3%5Ex

For u=2, 

3%5Ex=2

Take logs of both side:

log%283%5Ex%29=log%282%29

Use the principle of logs on the left that]
allows you to move an exponent in front of
the log:

xlog%283%29=log%282%29

Divide both sides by log%282%29

x=log%282%29%2F%28log%283%29%29

Use your calculator:

x=0.6309297536

For u=1%2F3, 

3%5Ex=1%2F3

Multiply through by 3 to clear of fractions:

3%2A3%5Ex=1

Write 3 as 3%5E1

3%5E1%2A3%5Ex=1

Add exponents on the left:

3%5E%281%2Bx%29=1

Write the 1 on the right as 3%5E0

3%5E%281%2Bx%29=3%5E0

Since the bases are both the same, and positive
and not equal to 1, we may equate the
exponents:

1%2Bx=0

x=-1

So there are two solutions,

x=0.6309297536 and x=-1

Edwin