1-(tan(39°)^2)/1+(tan(39°)^2) 求

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1-(tan(39°)^2)/1+(tan(39°)^2) 求
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1-(tan(39°)^2)/1+(tan(39°)^2) 求
1-(tan(39°)^2)/1+(tan(39°)^2) 求

1-(tan(39°)^2)/1+(tan(39°)^2) 求
(cos(39°)^2 -sin(*39°)^2)/(cos(39°)^2+sin(39°)^2)
=cos78°/1=cos78°

1-(tan(39°)^2)/1+(tan(39°)^2) =cos^2(39)-sin^2(39)=cos(78)

=(1-sin39^2/cos39^2)/(1+sin39^2/cos39^2)=cos39^2-sin39^2=cos(39*2)=cos78

1-(tan(39°)^2)/1+(tan(39°)^2) =cos^2(39)-sin^2(39)=cos(78)