View Full Version : Logarthmic Equation
LynnM
Oct 5, 2010, 03:14 PM
Can someone just get me started on this question:
2(logx)^2 -3 -5logx = 0 (Solve for x.)
Not too sure how to begin this one. Thank you.
Unknown008
Oct 6, 2010, 01:51 AM
You make use of a substitution to make it easier.
Let y = log\ x
2y^2 -3 -5y = 0
Rearrange:
2y^2 -5y -3 = 0
Solve this. After that, replace back y by log x.
Post what you get! :)
LynnM
Oct 6, 2010, 07:50 AM
Okay, here it is:
2y^2 - 5y - 3 = 0
(2y + 1) (y - 3) = 0
y = -1/2, 3
Here is where it gets tricky for me. Since a base can never be negative I'm not sure what to do about the -1/2. Would it still be a plausible solution since the entire logx may be a negative or is considered to be the base? This is the next step I took. Not too sure about it though.
2(-1/2)^2 - 3 - 5(-1/2) = 0
1/2 - 3 + 2.5 = 0
0 = 0
and
2(3)^2 - 3 - 5(3) = 0
18 - 3 - 15 = 0
0 = 0
Am I at least somewhat correct? (crossing my fingers!)
Unknown008
Oct 6, 2010, 07:57 AM
Yes, but now, you must replace back y by log x, like this:
y = -\frac12
So,
log\ x = -\frac12
And:
y = 3
So,
log\ x = 3
Solve for x now :)
LynnM
Oct 6, 2010, 08:15 AM
Like this:
logx = -1/2
10^{-1/2} = x
x = 0.3162
and
logx = 3
10^3 = x
x = 1000
Then, plugged back into the equation:
2(log0.3162)^2 - 3 - 5log0.3162 = 0
0.5 - 3 + 2.5 = 0
0 = 0
and
2(log1000)^2 - 3 - 5log1000 = 0
18 - 3 - 15 = 0
0 = 0
So, because the 2 and the power 2 are placed outside the brackets surrounding the logx, they are applied after solving for log0.3162 and log1000, correct? And the because the 5 is not outside of any brackets it should be raised as a power then log1000^5 should be solved, correct?
Unknown008
Oct 6, 2010, 08:21 AM
Right! :)
Well done!
LynnM
Oct 6, 2010, 08:24 AM
Yesss! -raises arms in triumphant relief-! Thanks a billion!
sandford11
Oct 13, 2010, 12:26 AM
Good solutions there I like them.