`#DyHungg`
`a) (a+b+c)^2+(a+b-c)^2`
`=(a+b+c)(a+b+c)+(a+b-c)(a+b-c)`
`=a²+c²+b²+2ab+2bc+2ac+a²+b²+c²+2ab-2ac-2bc`
`=2a²+2b²+2c²+4ab`
.......................
`b) (a+b)³-3(a+b)².b+3(a+b)b²-b³`
`=a³+3a²b+3ab²+b³-3b(a²+2ab+b²)+b²(3a+3b)-b³`
`=a³+3a²b+3ab²+b³-3a²b-6ab²-3b³+3ab²+3b³-b³`
`=a³`
.............
`c) (a²-b²)(a²+ab+b²)(a²-ab+b²)`
`=(a^4+a³b-ab³-b^4)(a²-ab+b²)`
`=a^6-a^5b+a^4b^2+a^5b-a^4b^2+a^3b^3-a³b³+a²b^4-ab^5-a^2b^4+ab^5-b^6`
`=a^6-b^6`