8 1×89=7209=8×﹙8+ 1﹚× 100+ 1×9

73×77=562 1=7×﹙7+ 1﹚× 100+3×7

45×45=2025=4×﹙4+ 1﹚× 100+5×5

64×66=4224=6×﹙6+ 1﹚× 100+4×6

(2) 2 is a quadratic power.

Let these two digits be (10n+a) and (10n+b) respectively, where a+b= 10.

∴( 10n+a)×( 10n+b)= 100n∧2+﹙a+b﹚× 10n+ab

= 100n∧2+ 100n+ab

=n﹙n+ 1﹚× 100+ab

I made it myself!