$\begin{array}{l}
16)\,\,\,a)\,\,{x^2} + 2x + 1 = {\left( {x + 1} \right)^2}\\
b),\,\,c)\,\,Em\,\,\,lam\,\,tuong\,\,tu.\\
d)\,\,\,\,{x^2} - x + \frac{1}{4} = {x^2} - 2.\frac{1}{2}x + {\left( {\frac{1}{2}} \right)^2} = {\left( {x - \frac{1}{2}} \right)^2}/\\
17)\,\,\,{\left( {10a + 5} \right)^2} = {\left( {10a} \right)^2} + 2.10a.5 + {5^2}\\
= 100{a^2} + 100a + 25 = 100a\left( {a + 1} \right) + 25.\\
{25^2} = {\left( {10.2 + 5} \right)^2} = 100.2\left( {2 + 1} \right) + 25 = 600 + 25 = 625.\\
Cac\,\,\,cau\,\,khac\,\,em\,\,lam\,\,tuong\,\,tu.\\
18)\,\,\,a)\,\,{x^2} + 6xy + .... = {\left( {.... + 3y} \right)^2}\\
\Leftrightarrow {x^2} + 2.x.3y + ...... = {\left( {.... + 3y} \right)^2}\\
\Leftrightarrow {x^2} + 6xy + {\left( {3y} \right)^2} = {\left( {x + 3y} \right)^2}.
\end{array}$
