Since, D is the mid-point of Bc, therefore by section formula, we have
`OD=(OB+OC)/(2)`
`implies OB+OC=2OD` . . . (i)
Similarly, `OC+OA=2OE` . . (ii)
and `OB+OA=2OF` . . . (iii)
On adding Eqs. (i), (ii) and (iii), we get
`2(OA+OB+OC)=2(OD+OE+OF)`
`impliesOA+OB+OC=OD+OE+OF` Hence proved.