[tex] \\\\\\ Bonjour; \\\\\\ sin^2(x) = cos^2(x) \times \dfrac{sin^2(x)}{cos^2(x)} = (1 - sin^2(x))tg^2(x) \\\\\\ = tg^2(x) - sin^2(x) tg^2(x) \\\\\\ donc \ : sin^2(x) + sin^2(x)tg^2(x) = tg^2(x) \\\\\\ donc \ : sin^2(x)(1 + tg^2(x)) = tg^2(x) \\\\\\ donc \ : sin^2(x) = \dfrac{tg^2(x)}{1+tg^2(x)} \ . [/tex]