Cos(a+b)= cosacosb - sinasinb
cos(a-b) = cosacosb + sinasinb
cos(8pí/15-pí/5) =cos(5pí/15) =cos(pí/3)=1/2
(x-5)²=(x-8)²
x² - 10x + 25 = x² - 16x + 64
6x = 39
x= 6,5
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(-x-5)*(2x+4)=0
-x-5 = 0
x₁= -5
2x+4=0
x₂= -2
Y=7-3x
4x-2(7-3x)=6
4x-14+6x=6
10x=20
x=2
X2-3x+9/(х+3)(х2-3х+9) - х2+18/х2+27=х2-3х+9-х2-18/(х+3)х2-3х+9)=-9-3х(х+3)(х2-3х+9)=
=-3(х+3)/(х+3)(х2-3х+9)= 3/х2-3х+9