Using your subroutine (2.1) and a code fragment similar
to that given for (1.2), write a main program which solves
Poisson's equation.
Notes: Be sure to integrate backwards from 
(u=1) to
r=0 (u=0): you'll have to change the order of the loop and the
values of both h and dk! Also, remember the boundary
conditions:
and 
.