第1个回答 2011-10-22
cosa+isina (cosa+isina)(cosb-isinb) cosacosb+sinasinb+i(sinacosb-cosasinb)
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cosb+isinb (cosb+isinb)(cosb-isinb) cosbcosb+sinbsinb
所以cos(b-a)=cosacosb+sinasinb