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Prove the following

anil86

New member
Nov 1, 2013
10
Please view attachment!!!Image0350.jpg
 

MarkFL

Administrator
Staff member
Feb 24, 2012
13,775
We are given:

\(\displaystyle \sin\left(\theta+i\phi \right)=\tan(x)+i \sec(x)\)

Using the angle-sum identity for sine on the left, we have:

\(\displaystyle \sin(\theta)\cosh(\phi)+i \cos(\theta)\sinh(\phi)=\tan(x)+i \sec(x)\)

This implies:

\(\displaystyle \sin(\theta)\cosh(\phi)=\tan(x)\)

\(\displaystyle \cos(\theta)\sinh(\phi)=\sec(x)\)

Now, we have the following double-angle identities:

\(\displaystyle \cos(2\theta)=\cos^2(\theta)-\sin^2(\theta)\)

\(\displaystyle \cosh(2\phi)=\cosh^2(\phi)+\sinh^2(\phi)\)

Hence:

\(\displaystyle \cos(2\theta)\cosh(2\phi)=\cos^2(\theta)\cosh^2( \phi)+\cos^2(\theta)\sinh^2(\phi)- \sin^2(\theta)\cosh^2(\phi)- \sin^2(\theta)\sinh^2(\phi)\)

Using the implications we drew above, we may write:

\(\displaystyle \cos(2\theta)\cosh(2\phi)=\cos^2(\theta)\cosh^2( \phi)+\sec^2(x)-\tan^2(x)- \sin^2(\theta)\sinh^2(\phi)\)

Using a Pythagorean identity, there results:

\(\displaystyle \cos(2\theta)\cosh(2\phi)=\cos^2(\theta)\cosh^2( \phi)+1- \sin^2(\theta)\sinh^2(\phi)\)

Using Pythagorean identities, we have:

\(\displaystyle \cos(2\theta)\cosh(2\phi)=\left(1-\sin^2(\theta) \right)\cosh^2( \phi)+1-\left(1-\cos^2(\theta) \right)\sinh^2(\phi)\)

We may arrange this as follows:

\(\displaystyle \cos(2\theta)\cosh(2\phi)=1+\left(\cosh^2( \phi)-\sinh^2(\phi) \right)+\left(\cos^2(\theta)\sinh^2(\phi)- \sin^2(\theta)\cosh^2(\phi) \right)\)

Using the hyperbolic identity and our previous results, we then find:

\(\displaystyle \cos(2\theta)\cosh(2\phi)=1+1+1=3\)