$\displaystyle \begin{array}{{>{\displaystyle}l}} Do\ \frac{3\pi }{4} < a< \pi \Rightarrow sina< 0\\ sin\ a=-\sqrt{1-cos^{2} a} =-\sqrt{1-\frac{9}{25}} =-\frac{4}{5}\\ sin\left(\frac{5\pi }{2} -a\right) =sin\frac{5\pi }{2} cosa-cos\frac{5\pi }{2} .sina=\frac{3}{5}\\ cos\left( a-\frac{7\pi }{2}\right) =cosa.cos\frac{7\pi }{2} +sina.sin\frac{7\pi }{2} =\frac{4}{5}\\ M=\frac{sin2a}{cos2a+1} =\frac{2sinacosa}{cos^{2} a-sin^{2} a} =\frac{24}{7} \end{array}$
