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根據二倍角公式以及萬能公式做 因為sina=2cosa,所以tana=2 sin2a+2sinacosa=sin2a+sin2a=2sin2a=2*[2tan(a)]/{1+[tan(a)]^2}=2*[2*2]/{1+2^2}=2*4/5=8/5
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