TL084 can be used as HPF and LPF to get a bandpass filter.
But how it can be used as a rectifier and envelope detector?
any example circuit and mathematical equations for determining the circuit parameters.
Please share i want to study those.

 

But how it can be used as a rectifier and envelope detector?

Since it's homework, I would suggest you do some research and try to learn something. It's not difficult to type "envelope detector" into Google. First result:

http://en.wikipedia.org/wiki/Envelope_detector

 

standard diode has drawback of Vf=0.7V. this ruins use for small signal (if signal is sufficiently large, those 0.7V may be insignificant). by adding an opamp one can build 'precision rectifier' and envelope detector that can work on mV signals.

http://en.wikipedia.org/wiki/Precision_rectifier
http://sound.westhost.com/appnotes/an001.htm