`e)`
`(a+b-c)^2 + (a-b+c)^2 - 2(b-c)^2`
` = [(a+b) - c]^2 + [(a-b) + c]^2 - 2 (b^2 - 2bc + c^2)`
` = (a+b)^2 - 2c(a+b) + c^2 + (a-b)^2 + 2c(a-b) + c^2 - (2b^2 - 4bc + 2c^2)`
`= a^2 + 2ab + b^2 - 2ac - 2bc + c^2 + a^2 - 2ab + b^2 + 2ac - 2bc + c^2 - 2b^2 + 4bc- 2c^2`
` = (a^2 + a^2) + (b^2 + b^2 -2b^2) + (c^2 + c^2 - 2c^2) + (2ab - 2ab) + (4bc - 2bc - 2bc) + (2ac - 2ac)`
` = 2a^2 + 0 + 0 + 0 + 0 + 0`
` = 2a^2`
Vậy `(a+b-c)^2 + (a-b+c)^2 - 2(b-c)^2 = 2a^2`
