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**Dyoni****Member**- Registered: 2013-05-12
- Posts: 1

Hi,

I have a probelm with this:

if log(3)= x and log(5)= y, then log(45√15) =...??

I tried this: log(45)+log(√15) = log(3).(3).(5)+½log(3).(5)

then.. log(3)+log(3)+log(5)+½log(3)+log(5)

and that's all equal to.. x+x+y+½(x+y) = 2x+y+½(x+y) = 2x√x+y√y

but the real answer is ½(5x+2y)

anyone can help me? please..I really confuse...

Thank's..

*Last edited by Dyoni (2013-05-12 16:20:35)*

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 102,393

Hi;

Which does not agree with your real answer...

Please check your "real answer," because when an answer does not agree with what I get it is almost certainly wrong. Okay, that is an exaggeration, it has a darn good chance of being wrong. Okay, that is stretching it too but check it anyway.

**In mathematics, you don't understand things. You just get used to them.****If it ain't broke, fix it until it is.** **A number by itself is useful, but it is far more useful to know how accurate or certain that number is.**

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**ElainaVW****Member**- Registered: 2013-04-29
- Posts: 482

Try wolfram to check that.

Enter "PowerExpand[log(45 sqrt(15))]" the substitutions will verify Bobby's answer.

*Last edited by ElainaVW (2013-05-12 17:48:30)*

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**John E. Franklin****Member**- Registered: 2005-08-29
- Posts: 3,588

the half thing seems right because log(400) is about 2.6 and 2.6/2=1.3 and log (20) sorta is 1.301, so that's good...

**igloo** **myrtilles** **fourmis**

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