p1a1v1=p2a2v2  or A1V1=A2V2
						

							Let us consider a tube AB of varing cross-section with water is flowing through that tube.
  At side A, area of cross-section be `A1`
                    velocity of fluid be `v1`
  At side B, area of cross-section be `A2`
                    velocity of fluid be `v2`
 and density of water be `d` and let `A1`>`A2`
 
Now v=distance travelled/time=l/t --->l=vt
---> Mass of fluid entered the tube at A in `t`seconds, m1=dV1=dAl=dA1v1t ----->(1)
---> Mass of fluid leaved the tube at B in `t`seconds, m2=dV2=dAl=dA2v2t ----->(2)

But mass of fluid which entered into the  tube is equal to the mass of fluid leaved the fluid.
 Therefore from equations (1) and (2)
                                       m1=m2
                                       dA1v1t=dA2v2t
                           Therefore  A1v1t=A2v2t
                           Therefore Av=constant
						

							Consider the mass of a fluid entering a at A in `t` seconds
                                    M1=density*A1*V1*t
Mass of the fluid leaving the tube at B in `t` seconds
                                  M2=A2*density*V2*t
We know that M1=M2,
   density*A1*V1*t=density*A2*V2*t
               A1*V1=A2*V2
             Therefore A1*V1=constant