Đáp án:
`a)A=x²+y²+z²+2xy+2yz+2xz`
`b)B=x²+y²+z²-2xy+2yz-2xz`
`c)C=x²+y²+z²-2xy-2yz+2xz`
`d)D=x²+2x+1-4xy-4y+4y²`
Giải thích các bước giải:
`a)A=(x+y+z)²`
`=[(x+y)+z]²`
`=(x+y)²+2.(x+y).z+z²`
`=x²+2xy+y²+2xz+2yz+z²`
`=x²+y²+z²+2xy+2yz+2xz`
`b)B=(x-y-z)²`
`=[(x-y)-z]²`
`=(x-y)²-2.(x-y).z+z²`
`=x²-2xy+y²-2xz+2yz+z²`
`=x²+y²+z²-2xy+2yz-2xz`
`c)C=(x-y+z)²`
`=[(x-y)+z]²`
`=(x-y)²+2.(x-y).z+z²`
`=x²-2xy+y²+2xz-2yz+z²`
`=x²+y²+z²-2xy-2yz+2xz`
`d)D=(x+1-2y)²`
`=[(x+1)-2y]²`
`=(x+1)²-2.(x+1).2y+(2y)²`
`=x²+2x+1-4xy-4y+4y²`
