I am thinking about developing applications for windows platform. I want to start with windows 10 as it is a future platform. If i develop an app targeting windows 10 will it work in windows 8.1& windows phone 8.1 ?
No, if you specifically target Windows 10 as the platform your apps won't work on Windows Phone 8.1 as APIs are needed that are not availiable in earlier Versions of the OS.
However if you target your app for WP8.1 it will work on Windows 10 for phones!