The coefficient ofa3b4c5 in (bc+ca+ab)6 is
60
[If n is a positive integer and a1a2a3........am ∈ C then
(a1+a2+a3+.....am)n=∑n!n1!n2!.......nm!a1n1a2n2a3n3.....amnm where n1n2n3....nm are all non-negative integers subject to the condition n1+n2+n3+.....+nm=n]
a3b4c5=(ab)x(bc)y(ca)z
=az+xbx+ycy+z
⇒z+x=3, x+y=4, y+z=5 ⇒x+y+z=6
then x=1, y=3, z=2
The co-efficient of a3b4c5=6!1!2!3!=60