1. (13-9)*(13+9)=88
2. (20-19)*(20+19)=39
3.(2,2-2,8)*(2,2+2,8)=-3
4.(3,5-3,7)*(3,5+3.7)=-1,44
5.(5/6-2/3)*(5/6+2/3)=0,25
6.(7/9-1/6)*(7/9+1/6)=187/324
7.(5/12-3/4)*(5/12+3/4)=-(7/18)
8.(3/10-4/5)*(3/10+4/5)=-(11/20)
9.(8/15-4/5)*(8/15+4/5)=-(16/45)
10. ((2+1/7)-(2+1/7))*((2+1/7)+(2+1/7))= 4+2/7