(ab + bc + ac)2 = [(ab + bc) + ac]2
= (ab + bc)2 + 2(ab + bc).ac + (ac)2
= (ab)2 + 2ab.bc + (bc)2 + 2ac.ab + 2ac.bc + (ac)2
= (ab)2 + 2ab2c + (bc)2 + 2a2bc + 2abc2 + (ac)2
= (ab)2 + (bc)2 + (ac)2 + 2a2bc + 2ab2c + 2abc2
= (ab)2 + (bc)2 + (ac)2 + 2abc(a + b + c)
(ab+bc+ac)2=ab2+bc2+ac2+2ab+2ac+2bc
k minh nha cô vừa mới bày hôm qua xong 3412364786153746574543155431534534
(ab+bc+ac)2= a2b2+b2c2+a2c2+2ab2c+2a2bc+2abc2