Let n be a positive integer. The expression (3x2−x2)n is expanded in ascending powers of x.
a. (i) Write the general term Tr+1 of the expansion in terms of n and r.
b. (ii) Show that the power of x in Tr+1 is 2n−3r.
c. (iii) For n=8, determine all integral values of r for which the term is independent of x.