#include "TTree.h"

void Min_work_sample(){
TFile *f_data = TFile::Open("ROOT_sample.root","update"); 
TTree* tree1 = (TTree*) f_data->Get("weightedTree");


double D_ETA;
double D_ETA_w;
double weight;

TBranch *newb = tree1->Branch("D_ETA_w",&D_ETA_w,"D_ETA_w/D"); 

tree1->SetBranchAddress("weight", &weight);
tree1->SetBranchAddress("D_ETA", &D_ETA);
Long64_t nentries = tree1->GetEntries();
 for(Long64_t i=0; i< nentries; i++){
	 tree1->GetEntry(i);
	 D_ETA_w = D_ETA * weight;
	 newb->Fill();
	 
	}

tree1->Write("");
}