Cos(a+b)= cosacosb - sinasinb
cos(a-b) = cosacosb + sinasinb
cos(8pí/15-pí/5) =cos(5pí/15) =cos(pí/3)=1/2
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1)x²/y³+1/x=(x³+y³)/xy³
2)x/y²-1/y+1/x=(x²-xy+y²)/xy²
3)(x²+2xy+y²+4xy):(x+y)/x=(x²+6xy+y²)*x/(x+y)
4)(x+y)(x²-xy+y²)/xy³*xy²/(x²-xy+y²)*(x+y)/x(x²+6xy+y²)=(x+y)²/xy(x²+6xy+y²)