The sum of the series 20c0−20c1+20c2−20c3+...20c10 is
S = 20C0−20C1+20C2+.......+20C10
S=20C20−20C19+...........+20C11+20C10
2S=20C0−20C1+............+20C9+20C10+20C10+20C10−20C11+....+20C20
= 20C10⇒S=12(20C10)