a)\(a^3+b^3+c^3-3abc\)
\(=\left(a+b+c\right)\left(a^2+b^2+c^2-ab-bc-ca\right)\)
b)\(\left(a+b+c\right)^3-a^3-b^3-c^3\)
\(=3\left(a+b\right)\left(b+c\right)\left(c+a\right)\)
cho tam giac abc goi d,e lan luot la trung diem ab,ac.g la giao diem be,cd.goi m doi xung vs c qua d,n doi xung vs b qua e vs i,k lan luot doi xung vs g qua d,e
a)cm n,m doi xung vs nhau qua a
b)tinh de,biet bc=3cm