`(a+b+c)^2 + (a+b-c)^2 - 2(a+b)^2`
`= [(a+b) +c)]^2 + [(a+b) - c]^2 - 2(a+b)^2`
`= (a+b)^2 + 2.(a+b).c + c^2 + (a+b)^2 - 2(a+b).c + c^2 - 2(a+b)^2`
`= [ (a+b)^2 + (a+b)^2] + [ 2(a+b).c - 2(a+b).c] + c^2 + c^2 - 2(a+b)^2`
`= 2(a+b)^2 + 2c^2 - 2(a+b)^2`
`= [ 2(a+b)^2 - 2(a+b)^2] + 2c^2`
`= 2c^2`