cos2x/根号2cos(x+π/4)=1/5,0
问题描述：
cos2x/根号2cos(x+π/4)=1/5,0
答
(cos2x)/√2cos(x＋π/4)＝(cos²x－sin²x)/[√2(cosxcos(π/4)－sinxsin(π/4))]＝[(cosx＋sinx)(cosx－sinx)]/(cosx－sinx)＝cosx＋sinx＝1/5即可.