Ta có: $cotx=\frac{cosx}{sinx}$, $cos^{2}x+sin^{2}x=1$ (Cái này tự chứng minh)
Ta có: $\frac{cos^{2}x-sin^{2}y}{sin^{2}xsin^{2}y}-cot^{2}xcot^{2}y=\frac{cos^{2}x-sin^{2}y-cos^{2}xcos^{2}y}{sin^{2}xsin^{2}y}=\frac{cos^{2}x(1-cos^{2}y)-sin^{2}y}{sin^{2}xsin^{2}y}=\frac{cos^{2}xsin^{2}y-sin^{2}y}{sin^{2}xsin^{2}y}=\frac{sin^{2}y(cos^{2}x-1)}{sin^{2}xsin^{2}y}=\frac{-sin^{2}xsin^{2}y}{sin^{2}xsin^{2}y}=-1$
=> Bt= -1
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