Ответ:

x = -п/4+пn,n∈Z

x = arctg(1/7)+ пn,n∈

Объяснение:

cos²x-7sin²x=3sin(2x)

cos²x-7sin²x-6*sinx*cosx=0|:cos²x≠0

1-7tg²x-6tgx=0|:(-1)

7tg²x+6tgx-1=0

D = 6²-4*7*(-1) = 36+28 = 64 = 8²

(tgx)12 = (-6±8)/(2*7)

(tgx)1 = -1

(tgx)2 = 1/7

a) tgx = -1

x = -п/4+пn,n∈Z

b) tgx = 1/7

x = arctg(1/7)+ пn,n∈Z