Let GP:a,ar,ar2,ar3,...
Now, given: 2ar=ar2+ar3
⇒2=r+r2
⇒r2+r−2=0
⇒(r+2)(r−1)=0
⇒r=−2,1[SinceGPis not constant so r=1 is rejected]
⇒r=−2
⇒S20−S18=1−ra(1−r20)−1−ra(1−r18)
⇒S20−S18=1−ra[r18−r20]
⇒S20−S18=8×3[218−220]
⇒S20−S18=24218(1−4)
⇒S20−S18=−215