Bài 3,
3 , (x-y)^{2}(x+y)^{2}=x^{2}-2xy+y^{2} -x^{2}-2xy-y^{2}=-4xy
4, (7x+1)^{2}-(x+7)^{2}=49x^{2} + 14x+1 - x^{2}-14x-49=48x^{2}-48=48(x^{2}-1)
5,
a^{2}+b^{2}+c^{2}-ab-ac-bc=0 \Rightarrow 2a^{2}+2b^{2}+2c^{2}-2ab-2ac-2bc=0\Leftrightarrow (a-b)^{2}+(b-c)^{2}+(a-c)^{2}=0
=> đpcm
6...