please i want to know when i mic a grand piano for the sack of the ambient what will be fed back what kind of ambient will i get
I'm don't quite understand your question. If you are asking what is the best technique for capturing the ambient sound of a piano, the answer is fairly imprecise. There are many techniques for capturing ambient sound, most having to do with using broad spectrum microphones placed several feet away from the piano. There are many factors to consider; for example lid up or down - wanting a brighter or deeper sound - positioning the mics evenly to avoid phase/delay issues - etc.
The best way to decide is to experiment. You'll want microhones with extended frequendy response as the piano produces a very wide frequency range.
Hope that at least gets you started.