(b) – 12
Let a and r be the first term and common ratio respectively of the given G.P.
Then a + ar = 12 ...(i)
ar2 + ar3 = 48 ....(ii)
⇒ \(\frac{ar^2(1+r)}{a(1+r)}\) = \(\frac{48}{12}\) (Dividing (ii) by (i))
⇒ r2 = 4 ⇒ r ± 2 ⇒ r = – 2
as the terms of the G.P. are alternately positive and negative.
Now a (1 + r) = 12 ⇒ a (1 – 2) = 12 ⇒ a = – 12.