# Logarithms problem

1. Oct 25, 2014

### Rectifier

Hey!
I have a question regarding a statement in my physics book. I dont see how
$$|H(f)|_{dB} = - 10log (1+( \frac{f}{f_B})^2)$$

approaches this equation below for big values on f.

$$|H(f)|_{dB} = - 20log ( \frac{f}{f_B})$$

EDIT: I am sorry if this is posted to a wrong sub-forum.

Last edited: Oct 25, 2014
2. Oct 25, 2014

### ShayanJ

When f is large, you can ignore 1 and so there only remains $-10 \log{(\frac{f}{f_B})^2}$. Now you can use the property $\log a^n=n\log a$ to get what you want.

3. Oct 25, 2014

### Rectifier

Oh gosh. Thank you for your help :)

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