tan(a+π/4)=k,则cos2a=?
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tan(a+π/4)=k,则cos2a=?tan(a+π/4)=k,则cos2a=?tan(a+π/4)=k,则cos2a=?tan(a+π/4)=(tana+tanπ/4)/(1-tanatanπ/
tan(a+π/4)=k,则cos2a=?
tan(a+π/4)=k,则cos2a=?
tan(a+π/4)=k,则cos2a=?
tan(a+π/4)
=(tana+tanπ/4)/(1-tanatanπ/4)
=(tana+1)/(1-tana)
=k
∴tana=(k-1)/(k+1)
cos2a
=cos²a-sin²a
=(cos²a-sin²a)/1
=(cos²a-sin²a)/(cos²a+sin²a)
分子分母同除cos²a得
=(1-tan²a)/(1+tan²a)
=(2k)/(k²+1)
手机提问的朋友在客户端右上角评价点【满意】即可
根据二倍角公式tan（2a）=(2tan a)/(1-tana*tana) 求出tan（2a+π/2）=-cot(2a) 即tan（2a）和cot（2a） 然后自己在草稿纸上画三角形草稿 可以求出cos2a=(2*k)/(k*k+1)
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