2) ..........=(8 - b)(8+b)
3) ........=(c-1.5)(c+1.5)
4) ........=(1.7-d)(1.7+d)
5) ..........=(8/9 -x)(8/9 +x)
6) .........=(10/11 -y)(10/11 +y)
7) .......=(z- 13/14)(z+13/14)
8) .......=(t - 20/21)(t+20/21)
9) ........=(5x-6)(5x+6)
10) .......= -(16-49y²)= - (4-7y)(4+7y)
11) ......=(0.8 - 1/3 z)(0.8+1/3 z)
12) .......=(2/5 t - 6)(2/5 t +6)