Large Numbers

Lancimusss8 10 Feb 2009 03:17

In the notes Andy started out by having 10^{2} in congruent to (-3)^{2} mod (m)

I understand this but when it goes to the next line it has 10^{4} congruent to 9^{2}

which this still makes sense to me but he has that congruent to (-4)^{2} which doesn't make sense to me.

I don't quite remember why he said this method is oK? can have a little explanation why this method is cool with math?