how is multiply of icap and jcap is 0 ?

we know that 

  P . Q = PQcos teta

since i^{^} is perpendicular to the j^{^}  =>   teta = 90

  i^ . j^  = i^ . j^ cos 90

  i^  .  j^  =  0  since cos 90 =  0

hope u understood :)

since, i and j are unit vectors along x and y axis respectively

x and y axis are perpendicular to each other

hence thita= 90 degree.

we know by scalar product of vectors,

vector p * vector q= PQ cos (thita)

i * j= cos 90

cos90= 0

hence i cap and j cap is zero.