(99)5=(100−1)5
Using binomial theorem, we have
(99)5=5C0(100)5+5C1(100)4(−1)+5C2(100)3(−1)2 +5C3(100)2(−1)3
= (100)5+5(100)4(−1)+10(100)3+10(100)2(−1)3+5(100)+(−1)5
=10000000000−500000000+10000000−100000+ 500−1
=10010000500−500100001
=9509900499