+0  
 
0
837
5
avatar+386 

We have this:

 

logx+log(x+1)=log2

 

x=?

 

Can someone work me out every step? I have the answers in my book but the way it's put I just can't wrap my mind around every step they show...

 Jul 30, 2016

Best Answer 

 #4
avatar+129840 
+5

Note the reason for not be able to take a negative log.......

 

If we had

 

log (-2)     ......this would imply that  there is some "x" such that

 

10x  = -2

 

But notice that 10 raised to  any  "real" power is always positive....therefore.......there is no "real" power "x"   that we can raise 10 to that will result in a negative.......

 

 

 

cool cool cool

 Jul 30, 2016
 #1
avatar
0

Solve for x:
log(x)+log(x+1) = log(2)

 

log(x)+log(x+1) = log(x (x+1)):
log(x (x+1)) = log(2)

 

Cancel logarithms by taking exp of both sides:
x (x+1) = 2

 

Expand out terms of the left hand side:
x^2+x = 2

 

Add 1/4 to both sides:
x^2+x+1/4 = 9/4

 

Write the left hand side as a square:
(x+1/2)^2 = 9/4

 

Take the square root of both sides:
x+1/2 = 3/2 or x+1/2 = -3/2

 

Subtract 1/2 from both sides:
x = 1 or x+1/2 = -3/2

 

Subtract 1/2 from both sides:
x = 1 or x = -2

 

log(x)+log(x+1) ⇒ log(-2)+log(1-2) = 2 i π+log(2) ≈ 0.693147+6.28319 i
log(2) ⇒ log(2) ≈ 0.693147:
So this solution is incorrect

 

log(x)+log(x+1) ⇒ log(1)+log(1+1) = log(2) ≈ 0.693147
log(2) ⇒ log(2) ≈ 0.693147:
So this solution is correct

 

The solution is:
Answer: |x = 1

 Jul 30, 2016
 #2
avatar+129840 
+5

logx+log(x+1)=log2    

 

Remember that if we have log a + log b, we can write this as log [a * b]........so....

 

log x + log (x + 1)   =  log [ x (x + 1) ]  =  log [x ^2 + x]

 

So

 

log [ x^2 + x  ]    = log 2         we can just forget the logs and solve this equation :

 

x^2 + x  = 2      subtract 2 from both sides

 

x^2 + x  - 2  = 0         factor

 

(x -1) (x + 2)   = 0       and setting each factor to 0, x = 1  or x = -2

 

Since we can't take the log of a negative number, x = - 2 is not a solution

 

Check  x = 1

 

log (1) + log (1 + 1) =  log 2 ?

 

0 + log 2   = log 2

 

log 2 = log 2       OK!!!

 

 

 

cool cool cool

 Jul 30, 2016
 #3
avatar+386 
+5

I'll write here the solution included in my book:

 

logx + log(x+1) = log2

 

logx(x+1)= log2

 

logx(x+1)-log2=0

 

and then

 

log(x(x+1)/2)=0  (can someone explain me this step plz)

 

x(x+1)/2= 10exp0=1  (I feel this is some basic log law but I can't remember what it is exactly, help haha)

 

Then they multiply each side by 2 to get rid of the /2:

 

x^2+x-2=0  (why do they end up with "x^2" and "-2" when they just get rid of the /2?)

 

then they say "logx(x+1)=log2" means that "x(x+1)=2" and that "x^2+x-2=0"

 

Then they find rationnal roots for "x^2+x-2=0" which are "x=1" and "x=-2" (that's fine for me)

 

Seems that "x=1" makes sense because "log1+ log(1+1)=log2"

 

but that "x=-2" because " log of a negative number is undefined" (can someone refresh me on that).

 

Thanks a lot guys!

 Jul 30, 2016
 #5
avatar+9665 
0

logx(x+1)-log2=0-->log(x(x+1)/2)=0

\(\color{aqua}\boxed{\color{red}\log a - \log b = \log \dfrac{a}{b}}\)

\({\color{aqua}\text{So that }}\color{aqua}\log x(x+1)-log2 = \log \dfrac{x(x+1)}2\)

MaxWong  Jul 31, 2016
 #4
avatar+129840 
+5
Best Answer

Note the reason for not be able to take a negative log.......

 

If we had

 

log (-2)     ......this would imply that  there is some "x" such that

 

10x  = -2

 

But notice that 10 raised to  any  "real" power is always positive....therefore.......there is no "real" power "x"   that we can raise 10 to that will result in a negative.......

 

 

 

cool cool cool

CPhill Jul 30, 2016

3 Online Users

avatar