Question

помогите один пример!!!

Answer 1

$1-sin^2B= sin^2B+cos^2B-sin^2B=cos^2B\\cos^2B-1=cos^2B-sin^2B-cos^2B=-sin^2B\\\frac{1-sin^2B}{cos^2B-1}=\frac{cos^2B}{-sin^2B}=-ctg^2B$