1)(x+y)(x-y)=x²-y²
2)(n-m)(n+m)=n²-m²
3)(k-2)(k+2)=k²-2²=k²-4
4)(3-c)(3+c)=3²-c²=9-c²
5)(4+b)(4-b)=4²-b²=16-b²
6)(a-7)(a+7)=a²-7²=a²-49
7)(1/7+x)(1/7-x)=1/7²-x²=1/49-x²
8)(a-2/9)(a+2/9)=a²-2/9²=a²-4/81
9)(5/6+m)(5/6-m)=5/6²-m²=25/36-m²
10)(0,4+n)(0,4-n)=0,4²-n²=4/25-n²
11)(k+1,1)(k-1,1)=k²-1,1²=k²-1,21
12)(d-2,2)(d+2,2)=d²-2,2²=d²-4,84