Given, m is the A.M. of l&n.
⇒m=2l+n...(1)
G1,G2,G3 are G.M. between l&n.
⇒l,G1,G2,G3,n are in G.P.
⇒n=l(r)4⇒r4=ln...(2)
⇒G1=lr,G2=lr2,G3=lr3
⇒G14+2G24+G34=l4r4+2×l4r8+l4×r12
=l4r4[1+2r4+r8]
=l4×ln[1+r4]2=l3n×[1+ln]2
=l3×n×l2(l+n)2
=ln×(2m)2 (From (1))
=4lm2n