an =a1q^(n-1)a1.a2.a3=216(a1)^3.q^3 =216a1q =6 (1)a1+a2+a3 =26a1+a1q+ a1q^2 =26 a1+6 +6q=26 ( a1q =6)a1+6q=20a1 = 20-6q (2)from (1) and (2)(20-6q)q =63q^2-10q+3=0(3q-1)(q-3)=0q=1/3a1=18lim(n-> ∞) Sn ...