Cos(a+b)= cosacosb - sinasinb
cos(a-b) = cosacosb + sinasinb
cos(8pí/15-pí/5) =cos(5pí/15) =cos(pí/3)=1/2
1) 3(1+2ху)(1-2ху)=3(1-4х^2y^2)=3-12x^2y^2
2)(x^2-y^3)^2=x^4-2x^2y^3+y^6
3)(a+b)^2-(a-b)^2=a^2+2ab+b^2-a^2+2ab-b^2=4ab
2xy*5y-3y*3x^2=10xy^2-9x^2y=xy(10y-9x)
16х²-9-36х²+12х-1+18=0
-16х²+12х+8=0
16х²-12х-8=0
4(4х²-3х-2)=0
Д=9-4(4*(-2))=41
Х1= (3+√41)/8
Х2=(3-√41)/8
Ответ посчитаешь)