Miscellaneous typos and minor errors

Rochester/setSeries6CompTests/ur_sr_6_18.pg  Change "d to k" to "d to j" (there is no "k")
Rochester/setSeries6CompTests/benny_ser3.pg  Fixed typo (removed "not")
ma123DB/set10/s11_3_3.pg  Fixed typo ("x" --> "n")
Rochester/setVMultIntegrals3Appl/UR_VC_9_9.pg  Fixed typo (sign error)
Michigan/Chap7Sec4/Q57.pg  Fixed typo in solution
Michigan/Chap1Sec2/Q23.pg  Fixed perl error "uninitalized value $ans2"
maCalcDB/setIntegrals18Improper/ur_in_18_7.pg  The function needs to be continuously differentiable
Dartmouth/setStewartCh18S3/problem_5.pg
  Added line breaks before part A, use built-in $PI, and minor whitespace changes in the source
Dartmouth/setStewartCh18S3/problem_3.pg  Added $BR to clarify roles of input blanks; added units
UVA-Stew5e/setUVA-Stew5e-C01S01-Functions/1-1-24.pg  Switched to standard helpLink for interval notation