Упростите ctgβ - cosβ-1/sinβ
Ctgβ - (cosβ-1 / sinβ) => (1 / tgtβ) - (cosβ-1 / sinβ) => (cosβ / sinβ) - (cosβ-1/sinβ) => (Cosβ - cosβ + 1) / sinβ => 1 / sinβ => cscβ