6aq=a+a(q^2)+76aq - a - aq^2 = 7a(6q-1-q^2) =7[a(1-q^3)]/(1-q)=7(a(1-q^3))/(1-q)=7a(1-q)(1+q+q^2)/（1-q） =7a(1+q+q^2) = 7a(6q-1-q^2) = a(1+q+q^2)a(6q-1-q^2-1-q-q^2) =0a(5q-2-2q^2) = 0a = 0 当a = 0q无...