Bug fixes for several Rogawski problems

For WHFreeman/Rogawski problems:
8.1.13: don't use degenerate intervals like [3,3]
10.7.14: make sure the function is defined at x=0
9.4.9 and 5.8.35:  % changed to $PERCENT
9.5.17 and 9.5.21: webwork couldn't generate enough points sometimes
3.4.27 and 5.8.14: miscategorized