# Logarithmic equations help

1. Aug 23, 2008

### fr33pl4gu3

3e3y-6 = 2x2-1
ln3e3y-6=ln2x2-lne
ln3e3y-6=ln2x2/lne
3e3y-6=2x2/e
3e3y-6*e=2x2
e3y-6=(2/3)x2
3y-6=ln((2/3)x2)
3y=ln((2/3)x2)+6
y=(1/3)ln((2/3)x2)+2

Last edited: Aug 23, 2008
2. Aug 23, 2008

### HallsofIvy

Staff Emeritus
Re: Logarithm

Be carefule with parentheses: you mean ln3e3y-6=ln(2x2-1) because you are taking the log of the entire right side: 2x2- 1.

You see? Now you are treating that "-1" as if it were outside the logarithm: it isn't.

And even if it were, "ln a- ln b" is NOT "ln a/ln b" it is ln(a/b)

If you reverse what you did in the first step you should just come back to the first step! You don't because you have done it wrong.
ln(3e3y- 6)= ln(e3y- 6)+ ln 3= 3y- 6+ ln 3
3y- 6+ ln 3= ln(2x2- 1)

Solve that for y.

No, make the corrections I suggested.

3. Aug 23, 2008

### fr33pl4gu3

Re: Logarithm

ln(3e3y- 6)= ln(e3y- 6)+ ln 3= 3y- 6+ ln 3
3y- 6+ ln 3= ln(2x2- 1)
3y-6 = ln((2x2-1)/3)
3y = ln ((2x2-1)/3) + 6
y=(1/3)ln((2x2-1)/3)+2

4. Aug 23, 2008

### rootX

Re: Logarithm

Yes, that's what I got

5. Aug 23, 2008

### fr33pl4gu3

Re: Logarithm

New Question:

log8(9y + 14) = 6x6-11
9y+14 = 86x6-11

The next step is it by entering this step??

6. Aug 23, 2008

### snipez90

Re: Logarithm

If you're solving for y again then yes that is correct as a next step.

7. Aug 23, 2008

### fr33pl4gu3

Re: Logarithm

the next 2 step will be:

ln 9y + 14 = ln86x6-11
ln 9y + 14 = 6x6-11 ln8

8. Aug 23, 2008

### fr33pl4gu3

Re: Logarithm

log8(9y + 14) = 6x6-11
9y+14 = 86x6-11
ln (9y + 14) = ln86x6-11
ln (9y + 14) = (6x6-11) ln8
ln 9y + ln 14 = (6x6-11)ln8
ln9y=(6x6-11)ln8 - ln 14
y=((6x6-11)ln8 - ln 14)/ln9

Solve the equation of y, i wonder if this is right?

9. Aug 23, 2008

### HallsofIvy

Staff Emeritus
Re: Logarithm

This is NOT the equation you gave before! Before it was ln(3e3y- 6)

And if the "-6" was NOT in the exponent this is wrong. Did you mean "ln(3e3y- 6"?

10. Aug 23, 2008

### tiny-tim

Hi fr33pl4gu3!
No … ln (9y + 14) is not the same as ln 9y + ln 14, is it?

btw you should learn the very useful formula lnab = lnb/lna