a) 1+ (-2)+3+(-4)+...+19+(-20)
= [1+(-2)]+[3+(-4)]+...+[19+(-20)]
=(-1)+(-1)+....+(-1)
=-1.10
=-10
b) 1-2+3-4+...+99-100
=(1-2)+(3-4)+...(99-100)
=(-1)+(-1)+...+(-1)
=-1.50
=-50
c) 2-4+6-8+...+48-50
=(2-4)+(6-8)+...+(48-50)
=-2+(-2)+...+(-2)
=-2.25
=-50