a) S1= 1+(-2)+ 3+(-4)+...+2019+(- 2020)
S1=(1-2)+(3-4)+...+(2019-2020)
S1=-1-1+...+(-1)
S1=1009.(-1)
S1=-1009
b) S2= (-2)+4+(-6)+8+...(-2018)+2020
S2=(2+4)+(-6+8)+...+(-2018+2020)
S2=-2-2+...+(-2)
S2=504.(-2)
S2=-1008
c) S3=(-2020)+(-2019)+(-2018)+...+2018+2019+2020
S3=(-2020+2020)+...(-2019+2019)+(-2018+2018)
S3=0+0+...+0
S3=0