Which of the following mathematical system are semigroups? Which are groups? (vii) (G, *), where G={[a b -b a] |[a b -b a] not equal to [0 0 0 0] and a, b sum R} and * is the usual matrix multiplication. Which of the following mathematical system are semigroups? Which are groups? (vii) (G, *), where G={[a b -b a] |[a b -b a] not equal to [0 0 0 0] and a, b sum R} and * is the usual matrix multiplication