\(cos^2x=\dfrac{1}{tan^2x-1}\)
=>\(\dfrac{1}{1+tan^2x}=\dfrac{1}{tan^2x-1}\)
=>\(tan^2x+1=tan^2x-1\)
=>1=-1(vô lý)
=>\(x\in\varnothing\)
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\(cos^2x=\dfrac{1}{tan^2x-1}\)
=>\(\dfrac{1}{1+tan^2x}=\dfrac{1}{tan^2x-1}\)
=>\(tan^2x+1=tan^2x-1\)
=>1=-1(vô lý)
=>\(x\in\varnothing\)