19*21= (20-1) (20+1) =20^2 -1 = 400- 1 =399
99*101= (100 - 1) (100+1) = 100^2 -1 =10000 - 1 = 9 999
28*32= ( 30 - 2) ( 30+2) = 30 ^2 - 4 = 900 - 4 =896
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19*21= (20-1) (20+1) =20^2 -1 = 400- 1 =399
99*101= (100 - 1) (100+1) = 100^2 -1 =10000 - 1 = 9 999
28*32= ( 30 - 2) ( 30+2) = 30 ^2 - 4 = 900 - 4 =896