\[\boxed{\text{1172\ (1172).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[1)\ 19 \cdot 21 = (20 - 1)(20 + 1) =\]
\[= 400 - 1 = 399\]
\[2)\ 98 \cdot 102 =\]
\[= (100 - 2)(100 + 2) =\]
\[= 10000 - 4 = 9996\]
\[3)\ 2\frac{2}{3} \cdot 3\frac{1}{3} = \left( 3 - \frac{1}{3} \right)\left( 3 + \frac{1}{3} \right) =\]
\[= 9 - \frac{1}{9} = 8\frac{8}{9}\]
\[4)\ 7,9 \cdot 8,1 =\]
\[= (8 - 0,1)(8 + 0,1) =\]
\[= 64 - 0,01 = 63,99\ \]