why amminoa has higher dipole momentvthan …

Ans. 

This is due to the fact that dipole moment is a vector quantity and hence resultant dipole moment in a molecule will depend upon the direction of dipole moments of individual bonds.

Nitrogen has a lone pair of electrons and is more electronegative than hydrogen. So, in case of NH₃, the orbital moment due to the lone pair of electrons on nitrogen is in the same direction of the 3 N-H dipoles.Thus, the molecule has a net dipole moment and it is polar.

In NF₃, fluorine being more electronegative than nitrogen pulls the shared pair of electrons of the N-F bonds towards itself. Because of this, the orbital moment due to lone pair of electrons and the resultant dipole moment of 3 N-F bonds point in the opposite directions to that of lone pair of electrons. This lowers the dipole moment of NF₃.