1.
a) 69*71 = ((70-1)(70+1) = 70^2 - 1 = 4900-1=4899
б)89*91 = (90-1)(90+1) = 90^2 - 1 = 8100-1=8099
2.
а)58*62 = (60-2)(60+2)=60^2-2^2 = 3600 - 4=3596
б)42*38 = (40+2)(40-2) = 40^2-2^2 = 1600-4=1596
3.
а)0.49*0.51 = (0,5-0,1)(0,5+0,1) = 0,5^2 - 0,1^2 = 0,25 - 0,01=0,2499
б)0.67*0.73 = (0,7-0,03)(0,7+0,03) = 0,7^2 - 0,03^2 = 0,49 - 0,0009=0,4891
4.
а) 10(целых) 1/7*9(целых) 6/7 = 71/7*69/7 = (70/7+1/7)(70/7-1/7) =
=(70/7)^2 - (1/7)^2 = 4900/49 - 1/49 =4899 /49
б)99(целых) 2/3*100 1/3
= 299/3*301/3 = (300/3 -1/3)(300/3+1/3) =
= (300/3)^2 - (1/3)^2 = 90000/9 - 1/9 = 89999/9