QR question

[sbesbesbe]

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Hi there SDN

I do have a question on QR...
sin(2x-30)=cos(4x-60)
...
sin(2x-30)=cos(120-2x)
BUT
why does it become
4x-60=120-2x ? (why is 2x-30 doubled)?

please don't laugh if this is supposed to be an easy question...

 

[Mr Dental]

Say A=sin(2x-30).
Then A=cos(4x-60) and A=cos(120-2x)
A=A, so cos(4x-60)=cos(120-2x).
cosine on both sides cancel out, so 4x-60=120-2x.

Remember, a=b, a=c, then b=c.

 

[Troyvdg]

EDIT: someone just posted before me

 

[sbesbesbe]

Say A=sin(2x-30).
Then A=cos(4x-60) and A=cos(120-2x)
A=A, so cos(4x-60)=cos(120-2x).
cosine on both sides cancel out, so 4x-60=120-2x.

Remember, a=b, a=c, then b=c.

thanks a lot!