左边=tan^2(α)-1/ tan^2(α) = sin^2(α)/cos^2(α) - cos^2(α)/sin^2(α) = (sin^4(α)-cos^4(α)) / (cos^2(α) * sin^2(α)) = (sin^2(α) + cos2^(α))*(sin^2(α) - cos^2(α)) / (cos^2(α) * sin^2(α)) = ...