R=17k, C=15n

The lowpass is basically a voltage divider, hence Vout=Vin*Z/Xc

rearrange: Vout/Vin=Z/Xc

The corner frequency is where the voltage drops by sqrt(2) (and power by half):

1/sqrt(2)=Z/Xc

reactance Xc is 1/wC

impedance Z:

Z=17k-jXc=17k-j(1/wC)

substitute:

1/sqrt(2)=(17k-j(1/wC)) / (-j(1/wC))

solve for w:

w=-j(sqrt(2)-2)*(1000/51)=-j1148.60

f=w/(2*pi)=182.806i

to polar: 182.806 Hz<90°

The actual corner frequency is 624.1 Hz

what am I doing wrong? how to do it properly?