Find d

johans

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Joined
Apr 20, 2022
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Hi,

My maths from 20 years back I have forgotten.

I need to find the value of d

1650489501905.png
FSPL = -93
f = 868000
GTX = 6
GRX =3
c = 299792458

I cant recall how to manipulate the formula to find d, please help
 
Last edited:
Hi,

My maths from 20 years back I have forgotten.

I need to find the value of d

View attachment 32272
FSPL = -93
f = 868000
GTX = 6
GRX =3
c = 299792458

I cant recall how to manipulate the formula to find d, please help
You essentially have
log10(d)=Alog_{10}(d) = A
where A is some constant.

What is 10log10(d)10^{log_{10}(d)}? (Hint: log10(x)log_{10}(x) is the inverse function of 10x10^x.)

-Dan
 
You essentially have
log10(d)=Alog_{10}(d) = A
where A is some constant.

What is 10log10(d)10^{log_{10}(d)}? (Hint: log10(x)log_{10}(x) is the inverse function of 10x10^x.)

-Dan
Maybe his/her difficulty is in finding "A". ?
 
Hi,

My maths from 20 years back I have forgotten.

I need to find the value of d

View attachment 32272
FSPL = -93
f = 868000
GTX = 6
GRX =3
c = 299792458

I cant recall how to manipulate the formula to find d, please help
Can you simplify

93=20log10(d)+20log10(868000)+20log10(4π299792458)63\displaystyle -93 = 20 \log_{10}(d) + 20 \log_{10}(868 000) + 20\log_{10}(\frac{4\pi}{299 792 458}) - 6 - 3

down to

93=20log10(d)+??\displaystyle -93 = 20\log_{10}(d) + ??
 
The basic idea is this definition of logarithms.

Given α>0, α1, and β>0,logα(β)=γ    β=αγ.\text {Given } \alpha > 0, \ \alpha \ne 1, \text { and } \beta > 0, \\ \log_{\alpha}(\beta) = \gamma \iff \beta = \alpha^{\gamma}.
(From that definition, topsquark’s hint follows directly, but I actually think it is easier to work from the definition itself.)

The Cat has given you one method, but I find it a bit easier to go

93=20log10(d)+20log10(868000)+20log10(4π299792948)36    84=20log10+20log10(3.472106π2.997,929,480108)    log10(d)+log10(3.472π2.997,929,480102)=8420    log10(d)2log10(3.472π2.997,929,480)=4.2.-93 = 20 \log_{10}(d) + 20 \log_{10}(868000) + 20 \log_{10} \left ( \dfrac{4 \pi}{299792948} \right ) - 3 - 6 \implies \\ - 84 = 20 \log_{10} + 20 \log_{10} \left ( \dfrac{3.472 * 10^6 * \pi}{2.997,929,480 * 10^8} \right ) \implies\\ \log{10}(d) + \log_{10} \left ( \dfrac{3.472 * \pi}{2.997,929,480} * 10^{-2} \right ) = -\dfrac{84}{20} \implies\\ \log_{10}(d) - 2 \log_{10} \left ( \dfrac{3.472 * \pi}{2.997,929,480} \right ) = - 4.2.
Now can you finish it up?
 
The basic idea is this definition of logarithms.

Given α>0, α1, and β>0,logα(β)=γ    β=αγ.\text {Given } \alpha > 0, \ \alpha \ne 1, \text { and } \beta > 0, \\ \log_{\alpha}(\beta) = \gamma \iff \beta = \alpha^{\gamma}.
(From that definition, topsquark’s hint follows directly, but I actually think it is easier to work from the definition itself.)

The Cat has given you one method, but I find it a bit easier to go

93=20log10(d)+20log10(868000)+20log10(4π299792948)36    84=20log10+20log10(3.472106π2.997,929,480108)    log10(d)+log10(3.472π2.997,929,480102)=8420    log10(d)2log10(3.472π2.997,929,480)=4.2.-93 = 20 \log_{10}(d) + 20 \log_{10}(868000) + 20 \log_{10} \left ( \dfrac{4 \pi}{299792948} \right ) - 3 - 6 \implies \\ - 84 = 20 \log_{10} + 20 \log_{10} \left ( \dfrac{3.472 * 10^6 * \pi}{2.997,929,480 * 10^8} \right ) \implies\\ \log{10}(d) + \log_{10} \left ( \dfrac{3.472 * \pi}{2.997,929,480} * 10^{-2} \right ) = -\dfrac{84}{20} \implies\\ \log_{10}(d) - 2 \log_{10} \left ( \dfrac{3.472 * \pi}{2.997,929,480} \right ) = - 4.2.
Now can you finish it up?
Your second line is missing a d.
Your third line need 10 to be subscript.
This is a bit confusing for someone who has pretty much admitted they've forgotten about logs.
 
Thanks Harry. I managed to pack a typo into each line. That may be a record even for me.

Second line should read

84=20log10(d)+20log10(3.472106π2.997,929,480108)    - 84 = 20 \log_{10}(d) + 20 \log_{10} \left ( \dfrac{3.472 * 10^6 * \pi}{2.997,929,480 * 10^8} \right ) \implies
Third line should read

log10(d)+log10(3.472π2.997,929,480102)=8420    \log_{10}(d) + \log_{10} \left ( \dfrac{3.472 * \pi}{2.997,929,480} * 10^{-2} \right ) = - \dfrac{84}{20} \implies
The fourth line should read

log10(d)2+log10(3.472π2.997,929,480)=4.2.\log_{10}(d) - 2 + \log_{10} \left ( \dfrac{3.472 * \pi}{2.997,929,480} \right ) = - 4.2.
I apologize johans.
 
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