99.9(2004个9)乘以99...9(2004个9)加上199...9(2004个9)
=99.9(2004个9)×99...9(2004个9)+99...9(2004个9)+100...0(2004个0)
=99.9(2004个9)×[99...9(2004个9)+1]+100...0(2004个0)
=99.9(2004个9)×100...0(2004个0)+100...0(2004个0)
=[99.9(2004个9)+1]×100...0(2004个0)
=100...0(2004个0)×100...0(2004个0)
=100...0(4008个0)
因此,末尾有4008个0