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double angle identity

Synonym: 
double-angle identity, double angle formula, double-angle formula, double angle formulae, double-angle formulae
Type of Math Object: 
Theorem
Major Section: 
Reference

Mathematics Subject Classification

26A09 no label found33B10 no label found

Comments

Also through Euler formula (equivalent to Moivre formula mentioned by Mr. Akrowne):
e^{2i\alpha}=cos{2\alpha}+isin{2\alpha} (1)
(e^{i\alpha})^2=(cos{\alpha}+isin{\alpha})^2=
=(cos{\alpha}^2-sin{\alpha}^2)+i(2sin{\alpha}cos{\alpha}) (2)
and equating.

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