1) 2.1*1.9=3.99
2) 4.02*3.98=15.9996
3) 19.8*20.2=399.96
4) 1.05*0.95=0.9975
5) (3*2/3)^2-(2*1/3)^2=(11/3)^2-(7/3)^2=121/9-49/9=72/9=8
6) (4*1/3)^2-(1*1/6)^2=(25/6)^2-(7/6)^2=625/36-49/36=576/36=16
используется тождество sin^2 x + cos^2 x = 1
и tg x = sin x / cos x