1-sin^2 a / cos^2 a - 1

[latex] \frac{1-sin^2 \alpha }{cos^2 \alpha -1} =- \frac{cos^2}{1-cos^2 \alpha } =- \frac{cos^2 \alpha }{sin^2 \alpha } =-ctg^2 \alpha [/latex]

соs^2 a/ (-sin ^2 a)= - ctg a