Bài 12
a, E = (2x+3)^2 - 2(2x+3)(2x+5) + (2x+5)^2
       = (2x+3-2x-5)^2
       = (-2)^2
       = 4
b, F = (x^2 + x + 1)(x^2 - x + 1)(x^2 - 1)
       = (x^2 - x + 1)(x+1)(x^2 + x + 1)(x-1)
       = (x^3 + 1)(x^3 - 1)
       = x^6 - 1
c, G = (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 - 2bc + 2ac - 2b^2 + 4bc - 2c^2
       = 2a^2 + 2b^2 + 2c^2 + 2ab - 2ab - 2ac + 2ac - 4bc + 4bc - 2b^2 - 2c^2
       = 2a^2

e, A = (x+y)^2 - (x-y)^2
       = (x+y-x+y)(x+y+x-y)
       = 2y.2x
       = 4xy
f, B = (a+b)^3 + (a-b)^3 - 2a^3
      = a^3 + 3a^2.b + 3ab^2 + b^3 + a^3 - 3a^2.b + 3ab^2 - b^3 - 2a^3
      = 6ab^2
g, C = 9^8.2^8 - (18^4 - 1)(18^4 + 1)
       = 18^8 - (18^8 - 1)
       = 18^8 - 18^8 + 1
       = 0 + 1
       = 1

Bài 14
a, 153^2 + 94.153 + 47^2
= 153^2 + 2.47.153 + 47^2
= (153 + 47)^2
= 200^2
= 40000
b, 126^2 - 152.126 + 5776
= 126^2 - 2.76.126 + 76^2
= (126 - 76)^2
= 50^2
= 2500
c, 3^8.5^8 - (15^4 - 1)(15^4 + 1)
= 15^8 - (15^8 - 1)
= 15^8 - 15^8 + 1
= 1