Given GP has first term unity

∴ a₁ = 1.

Hence the GP will become

⇒ 1,r,r²,…

As given 4a₂+5a₃ is least

∴ 4r + 5r² is least

We can say f(r) = 4r + 5r²

Now the given function will be least if df(r)/dr =0

∴

⇒ 8+10r=0

⇒ 8=-10r

⇒