Sn=1²-2²+3²-4²+5²-6²+...+99²-100²=(1-2)(1+2)+(3-4)(3+4)+(5-6)(5+6)+.+(99-100)(99+100)=-1-2-3-4-5-6-.-99-100=-(1+2+3+4+5+6+.+99+100)=-(1+100)*100/2=-101*50=-5050