If the first term of a G.P. a1, a2, a3,.........is unity such that
4a2 + 5a3 is least, then the common ratio of G.P. is
None of these
a1 = 1, a2 = r, a3 = r2, ..........
∴ 4a2 + 5a3 = 4r + 5r2
To be its minimum ddr(4r + 5 r2) = 0 ⇒ r = −25.