Trình bày lời giải:
Bài `2.`
`a)`
`(a-b+c)^2`
`=[(a-b)+c]^2`
`=(a-b)^2+2.(a-b).c+c^2`
`=a^2-2ab+b^2+2ca-2cb+c^2`
`=a^2+b^2+c^2-2ab+2ac-2bc`
`b)`
`(a+b-c)^2`
`=[(a+b)-c]^2`
`=(a+b)^2-2.(a+b).c+c^2`
`=a^2+2ab+b^2-2ac-2bc+c^2`
`=a^2+b^2+c^2+2ab-2ac-2bc`
`c)`
`(a-b-c)^2`
`=[(a-b)-c]^2`
`=(a-b)^2-2.(a-b).c+c^2`
`=a^2-2ab+b^2-2ac+2bc+c^2`
`=a^2+b^2+c^2-2ab-2ac+2bc`