Solving a Logarithm Equation by SubstitutionDate: 12/18/2007 at 15:37:54 From: Eugene Subject: Evaluating "x" for a log equation I have a "thinking" question which states: solve (x+1)^(log(x+1)) = 100(x+1) I recently learned logs and I find it difficult to apply it to solving equations. So far I divided both sides by (x+1). Therefore: (x+1)^(log(x+1))/(x+1) = 100 then (x+1)^(log(x+1)-1) = 100 then i applied log to the entire equation log[(x+1)^(log(x+1)-1)] = log100 then the power comes in front (log(x+1)-1)*log(x+1) = log100 this is the part where I'm not sure if I'm doing it correctly. I divided the entire equation by 1og therefore i get x*(x+1) = 100 x^2 + x = 100 x^2 + x - 100 = 0 I applied the quadratic formula and got 9.51249 and -10.51249 I tried to put these back into the original equation, but they don't agree, so clearly I went wrong somewhere, but I can't figure out where. Date: 12/18/2007 at 17:03:13 From: Doctor Peterson Subject: Re: Evaluating Hi, Eugene. You wrote: >then the power comes in front > > (log(x+1)-1)*log(x+1) = log100 > >this is the part where I'm not sure if I'm doing it correctly. I >divided the entire equation by 1og No! No! No! ;-) "Log" is not a number, so you can't divide by it. Even if it had been a number, you can't divide every factor by something! What you CAN do here is to notice that "log(x+1)" occurs twice, and in fact that is the only form in which x appears. So it can simplify things if you give that a name, say y = log(x+1) Now put that name in place of the expression: (y - 1) * y = log(100) That ugly log on the right isn't so nice--until you realize that this is a base ten log, and you can evaluate it easily. Now solve for y, and then you can find what x is from there. If you're new to logs, this is quite a tricky problem, and you did very well as far as you got. And doing that check at the end is great--too many students don't bother to check, so they never realize that their solution was nonsense--and so they never learn. Now you will. If you have any further questions, feel free to write back. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/ Date: 12/18/2007 at 17:43:37 From: Eugene Subject: Evaluating "x" for a log equation AH! Thanks a lot. I was confused into dividing the log out because in one example we did in class we had a scenario where: log[base 3]((5x+10/(x-1)) = log[base 3](3x+2) and the log[base 3] was divided to produce (5x+10)/(x-1) = (3x+2) The above is correct, right?... (I hope it is.....) Thanks again. Date: 12/18/2007 at 22:45:36 From: Doctor Peterson Subject: Re: Evaluating Hi, Eugene. Yes, that's correct. I almost took the time to point out the difference with this kind of step in my first reply, but decided to hold that for later. Thanks for asking. What you're doing here is like the sort of "canceling" you do by dividing both sides of an equation by the same thing; but it depends on a different fact. Just as you "undo" a multiplication by performing the inverse operation, division, here you are undoing the logs by doing the inverse operation, namely an exponential function. Canceling the logs is done by raising the base, 3, to the quantities on each side of the equation, and using the fact that 3^log_3(a) = a. As a result, if log(a) = log(b), then a = b. (This can also be expressed by saying that the log is a one-to-one function; this is equivalent to the fact that it has an inverse.) You can only undo an operation or function in this way when it is the last one performed (following the order of operations), so that it is "exposed" on the "outside" of the expression. The reason you couldn't do it in your problem is that the logs were not "on the outside"; the last operation on the left side was a multiplication, so raising 10 to that power would not have the desired effect. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/ |
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