Question

пожалуйста, помогите​

Answer 1

$tga=-\frac{3}{4},\; \; \;\frac{\pi}{2}\leq a\leq\pi\\\\ctga=\frac{1}{tga}=-\frac{4}{3}\\\\sina=\pm\sqrt{\frac{1}{1+ctg^2a}}=\pm\sqrt{\frac{1}{1+(-4/3)^2}}=\pm\sqrt{\frac{1}{1+16/9}}=\pm\sqrt{\frac{1}{25/9}}=\\=\pm\sqrt{9/25}= \pm3/5\\\\\frac{\pi}{2}\leq a\leq\pi\; \; =>sina=3/5\\\\cosa=\pm\sqrt{\frac{1}{1+tg^2a}}=\pm\sqrt{\frac{1}{1+(-3/4)^2}}=\pm\sqrt{\frac{1}{1+9/16}}=\pm\sqrt{\frac{1}{25/16}}=\\=\pm\sqrt{16/25}= \pm4/5\\\\\frac{\pi}{2}\leq a\leq\pi\; \; =>cosa=-4/5$