99×101×9999
=(100-1)×(100+1)×(10000-1)
=(100²-1²)×(10000-1)
=(10000-1)×(10000-1)
=(10000-1)²
=10000²-2×10000×1+1²
=100000000-20000+1
=999980001
99×101×9999
=(100-1)×(100+1)×(10000-1)
=(100²-1²)×(10000-1)
=(10000-1)×(10000-1)
=(10000-1)²
=10000²-2×10000×1+1²
=100000000-20000+1
=999980001