I have tried to formulate proof for derivative of cosine and this is what I ended up with:(adsbygoogle = window.adsbygoogle || []).push({});

Does anyone see any errors? And since I found this proof a bit long, does anyone know any shorter version which proves all the steps involved?

Step1

http://www.viewdocsonline.com/document/4bp3pb

Step2

http://www.viewdocsonline.com/document/8e1qd2

Step3

http://www.viewdocsonline.com/document/tx1cp3

Summary

http://www.viewdocsonline.com/document/5f54l8

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# Working at proof derivative cos

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