5, a, Ta có: $\underbrace{99...9}8=10 ^{2005}-2$
.................(2004 chữ số 9)
\Rightarrow $ A=(10 ^{2005}-2)^2=10^{2005}-4.10^{2005}+4$
\Rightarrow $ A= 10^{2005}(10^{2005}-4)+4$
\Rightarrow $ A=10^{2005}.\underbrace{99...9}6+4=\underbrace{99...9600...04}$
.....................(2004 c/s...