If `i` current flow in the larger coil, magnetic field produced at the centre will be perpendicular to the plane of larger coil.
Now the area vector of smaller coil which is perpendicular to the plane of smaller coil will make an angle `theta` with the magnetic field.
Thus flux `=vecB.vecA=(mu_(0)i)/(2a_(2))*pia_(1)^(2)*cos theta or M=(mu_(0)pia_(1)^(2) cos theta_(1))/(2a_(2))`