a,S=(1-3+32-33)+............+(396-397+398-399)
S=(-20)+...................+396.(1-3+32-33)
S=(-20)+................+396.(-20)
S=(1+34+........+396).(-20) chia hết cho 20(đpcm)
b,3S=3-32+33-34+..............+399-3100
3S+S=(3-32+33-34+.............+399-3100)+(1-3+32-33+...............+398-399)
4S=-3100+1
S=\(\frac{-3^{100}+1}{4}\)