8. ctga + ctgβ = \(\frac{sin(a + β)}{sin a sinβ}\)

ctga + ctgβ = \(\frac{cos a}{sin a}\) + \(\frac{cos b}{sin b}\) = \(\frac{cos a sin b + sin a cos b}{sin a sin b}\) = \(\frac{sin(a + b)}{sin a sin b}\) Тождество верно.