Question

Упростите выражение 1-1/sin^2a

Answer 1

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