\[\boxed{\text{826.}\text{\ }еуроки - ответы\ на\ пятёрку}\]
\[\textbf{а)}\ (100 + 1)^{2} = 10\ 000 +\]
\[\text{+}200 + 1 = 10\ 201\]
\[\textbf{б)}\ (100 - 1)^{2} = 10\ 000 -\]
\[- 200 + 1 = 9801\]
\[\textbf{в)}\ 61² = (60 + 1)^{2} = 3600 +\]
\[+ 120 + 1 = 3721\]
\[\textbf{г)}\ 199² = (200 - 1)^{2} =\]
\[= 40\ 000 - 400 + 1 = 39\ 601\]
\[\textbf{д)}\ 999^{2} = (1000 - 1)^{2} =\]
\[= 1\ 000\ 000 - 2000 +\]
\[+ 1 = 998\ 001\]
\[\textbf{е)}\ 702² = (700 + 2)^{2} =\]
\[= 490\ 000 + 2800 + 4 =\]
\[= 492\ 804\]
\[\textbf{ж)}\ 9,9² = (10 - 0,1)^{2} = 100 -\]
\[- 2 + 0,01 = 98,01\]
\[\textbf{з)}\ 10,2² = (10 + 0,2)^{2} =\]
\[= 100 + 4 + 0,04 = 104,04\]