Khan Academy

[kenzan8000]

use divide and conquer strategy for making calculation faster. e.g. 2^90 = 2^50 * 2^40; congruence modulo: A≡B (mod C), equivalence: A mod C=B mod C, (A + B) mod C = (A mod C + B mod C) mod C, (A * B) mod C = (A mod C * B mod C) mod C, A^B mod C = ( (A mod C)^B ) mod C