4 sin(2 pi - a), если a =7pi/6

4sin(2π-7π/6)=4sin5π/6=4*1/2=2

[tex]4sin(2 \pi - \frac{7 \pi }{6} )=4sin( \frac{12 \pi -7 \pi }{6} )=4sin (\frac{5 \pi }{6} )=4sin( \frac{ \pi }{6} )=4* \frac{1}{2} =2[/tex]