i was wondering where can i get the tasks which i have completed on iphone app and web app?
Unfortunately, that's not possible on our mobile apps yet, but we'll consider adding this option in the future. You're welcome to vote for this request in our Votebox: http://todoist.com/Vote_iOS/showProposal/7/
As for the web-app - on a free account, you can enter a project and then click on the circular icon in the bottom-right corner of your task list to see tasks completed in this project. On a Premium account, you can click on the Karma score in the top-right corner, then "View All Completed" to see all completion events from all projects, filter them by date, label or projects and visualize your productivity graphically.