How do you prove this identity: \ln|\frac{\sin x}{\cos x-1}|=\ln|\frac{\cos x+1}{\sin x}|

Phoebe Xiong

Phoebe Xiong

Answered question

2022-02-28

How do you prove this identity:
ln|sinxcosx1|=ln|cosx+1sinx|

Answer & Explanation

Tatiana Berg

Tatiana Berg

Beginner2022-03-01Added 4 answers

We have
|sinxcosx1|=|sinxcosx1cosx+1cosx+1|=|sinx(cosx+1)(cosx)21|
Now using (cosx)2+(sinx2)=1, we get
|sinxcosx1|=|sinx(cosx+1)(sinx)2|
=|cosx+1sinx|
=|cosx+1sinx|
kawaktutoriallxd

kawaktutoriallxd

Beginner2022-03-02Added 1 answers

Since 2ln|a|=ln(a2) we have:
ln|sin(x)cos(x)1|=ln|cos(x)+1sin(x)|
ln((sin(x)cos(x)1)2)=ln((cos(x)+1sin(x))2)
That implies equality of arguments of logarithms. Then it is simple trigonometry:
(sin(x)cos(x)1)2=(cos(x)+1sin(x))2
sin4(x)=(1cos2(x))2

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