cos ((3pi+x)/3)*cos((9pi+2x)/6))=-5/48*tg(2arctg1,5)

Упростим

cos ((3pi+x)/3)*cos((9pi+2x)/6))= cos(pi+x/3) *cos(3pi/2+x/3)=

=-cos(x/3)* sin(x/3) = -0,5sin(2x/3)

tg(2*arctg1,5) =  2tg(arctg1,5)/(1-tg²(arctg1,5)=      {по ф-ле tg 2-ного угла}

= 2*1,5/(1-1,5²) =  3/(-1,25) =  -2,4

-0,5sin(2x/3) = -5/48*(-2,4)

sin(2x/3) = -0,5

2x/3 = (-1)^k+1pi/6 +pi*k

2x= (-1)^k+1pi/2 +3pi*k

x= (-1)^k+1pi/4 +1,5pi*k     k€Z