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cho a(y+z)=b(x+z)=c(x+y) chung minh
(y-z)/a(b-c) =(z-x)/b(c-d) =(x-y)/c(a-b) voi a,b,c khac nhau va khac o
(y-z)/a(b-c) =(z-x)/b(c-d) =(x-y)/c(a-b) voi a,b,c khac nhau va khac o