If a=1+b+b2+b3+... to ∞, then write b in terms of a given that |b|<1.
a=1+b+b2+b3+...∞
a=11−b [Since, S∞=a1−r]
a(1−b)=1
a−ab=1
ab=a−1
b=a−1a
If x = 1 + a + a2 .......... to ∞ (|a|<1), y = 1 + b + b2 ......... to ∞ (|b| < 1), then Z = 1 + ab + a2 b2 + a3 b3..... to ∞ is