Giải thích các bước giải:

a, `(2x+3)^2 - 2(2x+3)(2x+5) + (2x+5)^2`

`= [ 2x + 3 - ( 2x + 5 ) ]^2`

`= ( 2x + 3 - 2x - 5 )^2`

`= (-2)^2`

`= 4`

b, `(x^2 + x + 1 )(x^2 - x + 1 )(x^2-1)`

`= (x^2 + x + 1)(x^2 - x + 1)(x-1)(x+1)`

`= (x-1)(x^2+x+1)(x+1)(x^2-x+1)`

`= ( x^3 - 1^3)(x^3 + 1^3)`

`= (x^3 - 1)(x^3 + 1)`

`= (x^3)^2 - 1^2`

`= x^6 - 1`

c, `(a + b - c)^2 + ( a - b + c )^2 - 2( b - c)^2`

`= a^2 + b^2 + c^2 + 2ab - 2ac - 2bc + a^2+b^2+c^2 - 2ab  + 2ac - 2bc - 2 ( b^2 - 2bc + c^2 )` `= a^2 + b^2 + c^2 + 2ab - 2ac - 2bc + a^2+b^2+c^2 - 2ab  + 2ac - 2bc - 2b^2 + 4bc - 2c^2 `

`= ( a^2 + a^2 ) + ( b^2 + b^2 - 2b^2 ) + ( c^2 + c^2 - 2c^2 ) + ( 2ab - 2ab ) + ( -2ac + 2ac ) + ( -2bc - 2bc + 4bc )`

`= 2a^2`