The trigonometric identity is .
Consider left hand side identity :
\ \
\ \ Apply pythagorean identity : .
= 1
= Right hand side identity.
Thus, .
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.![\"\"](\"/test/fckeditor/editor/plugins/fckeditor_wiris/integration/showimage.php?formula=aca9d95c5d8f0a947571e43a7c83bc7b.png\")
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\ \
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\ \
Apply pythagorean identity : ![\"\"](\"/test/fckeditor/editor/plugins/fckeditor_wiris/integration/showimage.php?formula=dc83b1e8c48b00bc686c6de9ca4758ea.png\")
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.