i have configured captcha module in my drupal 7 site.. but can't understand how to show a captcha in content type, so that while adding content user will have to complete the captcha.
Please check README.txt of the captcha module.
The configuration page is at admin/config/people/captcha,
where you can configure the CAPTCHA
module and enable challenges for the desired forms. You can also
tweak the image CAPTCHA to your liking.
Go to the configuration page - admin/config/people/captcha
Select the Default challenge type - Math or Image
In the form list, you can define which form has CAPTCHA, for example if you want to have it on the register form, select a challenge type (Math or Image) for the FORM_ID user_register_form
If you want to add CAPTCHA to a content type, for example, blog, add a new FORM_ID "blog_node_form" and click "Save Configuration".
By default, you will not see CAPTCHA on each form when you logged in as administrator, but you can change the option in the configuration page.