5 cos(3pi/2 + a) , если a = 7pi/6
5 cos ( \frac{3 \pi }{2} + \frac{7 \pi }{6} )=5 cos ( \frac{9 \pi }{6} + \frac{7 \pi }{6} )=5 cos ( \frac{16 \pi }{6} )=5 cos ( \frac{ \pi }{3} )=5* \frac{1}{2} =2.5[/tex]