# Low pass filter equation trouble

#### Kelko

Joined Oct 1, 2020
33
Hi so I’ve attached a pic of the question and and also my current attempt but my brain just isn’t working today and I can get the 1+ts on the top but I can’t figure out where to go for the bottom from here. Have I done something wrong so far? Any help would be great

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#### Papabravo

Joined Feb 24, 2006
19,264
The numerator is ((R2)(jωC) + 1)/jωC
The denominator is (R1+R2)(jωC) + 1)/jωC
The jωC in both numerator and denominator cancel out and I think you can see your way clear to the rest

#### Kelko

Joined Oct 1, 2020
33
See I could get to that but I’m still not sure where the R1+R2/R2 on the denominator comes from as well as the R2C

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#### Papabravo

Joined Feb 24, 2006
19,264
First we let jω=s
Then we let τ=R2C
Then rewrite

$$(R_1 + R_2)\;=\;\dfrac{R_1+R_2}{R_2}\cdot R_2$$

Can you see it now?

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#### Kelko

Joined Oct 1, 2020
33
Ahhhh okay thank you so much

#### Papabravo

Joined Feb 24, 2006
19,264