∑xy(x+y)= ∑x(y2+z2)= ∑x2(y+z)
∑x2(y+z)- ∑yz(z+y)=∑(x2-yz)(y+z)=0
∑x(y-z)2=∑xy(x+y)-6∏x= ∑x(y2+z2)-6∏x≥0
∑x(x+y)(x+z)= ∑x3+3∏x+∑xy(x+y)
∏(x+y)= ∑xy(x+y)+2∏x
(∑x)( ∑xy)= ∑xy(x+y)+3∏x
∏(x+y-z)= ∑xy(x+y)- ∑x3-2∏x
(∑x)( ∑x2)= ∑x3+∑xy(x+y)