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1-tg²x/1+tg²x=(1-sin²x/cos²x) :(1+sin²x/cos²x)= cos²x-sin²x/cos²x : cos²x+sin²x/cos²x=
cos²x-sin²x/cos²x* cos²x/1=cos²x-sin²x=1-sin²x-sin²x=1-2sin²x