cn=an(b1+b2+...+bn)+bn(a1+a2+...+an)-anbn
c1=a1b1+b1a1-a1b1=a1b1
c2=a2(b1+b2)+b2(a1+a2)-a2b2=a1b2+a2(b1+b2)+
c3=a3(b1+b2+b3)+b3(a1+a2+a3)-a3b3=a1b3+a2b3+a3(b1+b2+b3)
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cn=an(b1+b2+...+bn)+bn(a1+a2+...+an)-anbn
c1=a1b1+b1a1-a1b1=a1b1
c2=a2(b1+b2)+b2(a1+a2)-a2b2=a1b2+a2(b1+b2)+
c3=a3(b1+b2+b3)+b3(a1+a2+a3)-a3b3=a1b3+a2b3+a3(b1+b2+b3)
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