Производная = - 4 ln³ cosx* (1/cos x ) * sinx = - 4 ln³ ctg x
127у = 139
у = 139/127
х = (13 - 8у) : 7
х = (13 -1112/127 ) : 7
х = 539/127 :7
х = 77/127
Cos^2a - sin^2a = 2cos^2a - 1
cos^2a = 1/(1+tg^2a) = 1/(1+4) = 0.2
cos^2a - sin^2a = 0.4 - 1 = -0.6
2x+13y+10=0 |*6
6x-2y-11=0 |*-2
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12x+78y+60=0 x=2x+13*(-1)+10=0
-12x+4y+22=0 x=2x-13+10=0
82y+82=0 2x-3=0
82y=-82 2x=3
y=-1 x=3:2 x=1.5
Как-то так