Можно решить с использованием таблицы косинусов
y=cos(x+π/3) при х=а
а=π/6: y=cos(π/6+π/3)=cos((π+2π)/6)=cos(3π/6)=cos(π/2)=0
a=π/3: y=cos(π/3+π/3)=cos(2π/3)=-1/2
a=π/2: y=cos(π/2+π/3)=cos((3π+2π)/6)=cos(5π/6)=-√3/2
a=7π/6: y=cos(7π/6+π/3)=cos((7π+2π)/6)=cos(9π/6)=cos(3π/2)=0
14х-2(х-1)+3(2х+6)= 14х-2х+2+6х+18= 18х+20
18х+20=0
18х=-20
х= -20
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18