Câu 85: Cho x < 0. Chứng minh rằng: $$ \sqrt{\frac{-1+\sqrt{1+\frac{1}{4}\left(2^{x}-2^{-x}\right)^{2}}}{1+\sqrt{1+\frac{1}{4}\left(2^{x}-2^{-x}\right)^{2}}}}=\frac{1-2^{x}}{1+2^{x}} $$