99...9(n个9)×99...90(n个9)+199...9(n个9)

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99...9(n个9)×99...90(n个9)+199...9(n个9)
99...9(n个9)×99...90(n个9)+199...9(n个9)

99...9(n个9)×99...90(n个9)+199...9(n个9)
=99...9(n个9)×99...9(n个9)+2×99...9(n个9)+1
=[99...9(n个9)+1]²
=(100…0)² (n个0)
=100…0 (2n个0)