Система

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x-y=13 п/2;

3cos^x-12cosy+4=0

X=13π/2+y

3cos² (13π/2+y) - 12cosy+=0

3sin²y-12cosy+4=0

3-3cos²y-12cosy+4=0

cosy=a

3a²+12a-7=0

D=144+84=228

a1 = (-12-2√57) / 6⇒cosy = (-12-2√57) / 6<-1 нет решения

a2 = (-12+2√57) / 6⇒cosy = (-12+2√57) / 6

y1=-arccos (2√57-12) / 6⇒x1=13π/2-arccos (2√57-12) / 6

y2=+arccos (2√57-12) / 6⇒x2=13π/2+arccos (2√57-12) / 6