C2 Logarithms

Is it a valid thing to do to put log in front of every term of an equation or will this change what the equation means?.

Reply 1

C.L-A

Yer this is fine you can go from x^2 + 5 = 8 to 2logx +log5 =log8

Posted from TSR Mobile

Reply 2

Mackiemcmasher

Original post by C.L-A

Yer this is fine you can go from x^2 + 5 = 8 to 2logx +log5 =log8

Posted from TSR Mobile

Ok awesome thank you for the help.

Reply 3

Devillain

Please forget what the person before said. Remember one rule of logarithms: ln(xy)= lnx+lny?
ln(x+y) is not necessarily lnx+lny. You can put a log in front of every equation, in the way that
x^2+5=8 is like ln(x^2+5)=ln8. But just claiming that a sum inside a log is like a sum outside a log is incorrect... and fairly stupid as well.

Reply 4

RizK

Original post by C.L-A

Yer this is fine you can go from x^2 + 5 = 8 to 2logx +log5 =log8

Posted from TSR Mobile

No, you can't do that!
take the equation again:

x^2 + 5 = 8, yeah?

Now simplify: x^2 = 3.

If you log both sides now, 2logx = log3.

However by what you did, 2logx + log5 = log 8.

log3 is not equal to log8-log5.

What you did is wrong. You can't do that.

Reply 5

Mackiemcmasher

Original post by RizK

So why is this wrong and in what circumstances is it the correct thing to do? do you have to simplify all of the terms first?

Reply 6

RizK

Original post by Mackiemcmasher

So why is this wrong and in what circumstances is it the correct thing to do? do you have to simplify all of the terms first?

If you do something to one side of the equal sign you must do the same to the other therefore put the whole thing inside a bracket then log the whole thing. :smile: