Tnth term of an AP=A+(n-1)D

thus if d nos r in AP their reciprocals shud b in HP

thus ...................1/a=A+(p-1)D.......................................1

                             1/b=A+(q-1)D..........................................2

                             1/c=A+(r-1)D.....................................3

nw (2-3)equations ,(3-1)equations,(1-2)equationss

thus u get (2-3)equations as

(q-r)*D*bc=c-b

similarly u'll egt 4 d other equations ......then add all of them and a,b,c get cancelled thsu u get d final answer as zeroooooo