A) Prove that C* is a group under multiplication
B) let H={a+bi € G|a^2 + b^2 = 1}. Prove that H is a subgroup of C*.
C) Prove that the set of nth roots of unity Un is a subgroup of H
D) let G be the group of all real 2x2 matrices of the form (a,b, -b, a, where not both a and b are 0, under matrix multiplication. Show that C* and G are isomorphic.