已知sin(Pai/4-α)=5/13,α属于(0,Pai/4),求cos2α/cos(Pai/4 α)的值
要详细解析过程
cos(Pai/4 α) =cos(-a -Pai/4) =cos(Pai/4-α -Pai/2) =sin(Pai/4-α) =5/13; (1)
即√2/2(cosa -sina) =5/13
左右平方得:(cosa -sina)² =1-sin2a = 10/13 ---->sin2a =3/13
α属于(0,Pai/4),2a属于(0,Pai/2),cos2a >0
cos2a = 4√10 /13 (2)
cos2α/cos(Pai/4 α) =4√10 /5
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