I want to know about the directivity of thin dipole antenna of length 1.25λ for my assignment.Is there any formula related to its length?
I would imagine that by now, you have completed your homework. But for the sake of closing the topic, the directivity of a 1.25 wavelength, center fed (a very important qualifier!) dipole is ~3.27. Therefore the maximum possible gain is 5.15 dBi.
You can apply the general formulas for field at any distance from a center fed dipole and solve for the maxima. As they are rather lengthy, if you need the specific E or H field formulas, just post back as a comment and I will supply them.