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...
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.......
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
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!!!
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!
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.......