1 (4-b)*(4+b)
2 (c-7)*(c+7)
3 (0.2 -a)*(0.2 +a)
4 (x -2/3)*(x+2/3)
5 (2x -5)*(2x+5)
6(9c -8d)*(9c+8d)
7 (0.3x-0.5y)*(0.3+0.5)
8 (ab^2 - c^3d^4)*(ab^2+c^3d^4)
9 (2 ac - 3xy)*( 2ac+ 3xy)
10 ( x^12- y^11)*(y^12+y^11)
11 (a^6 -40)*(a^6+40)
12 (a^9 -7/8)*( a^9+7/8)