2cos2x=7cosx

2cos2x=7cosx 2cos2x-7cosx=0 2(cosx)^2-2(sinx)^2-7cosx=0 2(cosx)^2-2(1-(cosx)^2)-7cosx=0 2(cosx)^2-2+2(cosx)^2-7cosx=0 4(cosx)^2-7cosx-2=0 t=cosx 4t^2-7t-2=0 D=(-7)^2-4*4*(-2)=49+32=81 t1=(7+9):8=2         t2=(7-9):8=-1/4 cosx=2                                                         cosx=-1/4  корней нет, т.к. |cosx|<1                           x=пи-arccos(1/4)+2пиэн, эн принадлежит Z