100.2 × 99.8 how many steps is it necessary to calculate with the square difference

100.2 × 99.8 how many steps is it necessary to calculate with the square difference

100.2*99.8
=(100+0.2)*(100-0.2)
=100*100-0.2*0.2
=10000-0.04
=9999.96

Use the square difference to calculate
（-x+y）（-x-y）
（-x+y）（x+y）

What is this for?
=x^2-y^2
=y^2-x^2

Using the formula of square difference to calculate
7×2004－7×2000－7×3

=7*(2004-2000-3)
=7

Use the square difference to calculate the following questions
（a-b+c-d)(a+b-c-d); (3a+b-2)(3a-b+2)

(a-b+c-d)(a+b-c-d)=[(a-d)-(b-c)][(a-d)+(b-c)]=(a-d)²-(b-c)²=a²+b²+c²+d²-2ad+2bc(3a+b-2)(3a-b+2)=[3a+(b-2)][3a-(b-2)]=9a²-(b-2)²=9a²-b²+4b-4