Let A be an n × n matrix with det A = 1 and with all

QUESTION:

Let A be an n × n matrix with det A = 1 and with all elements of A integers.(a) Show that A?1 has only integer entries.________________(b) Suppose that b is an n-vector with only integer entries. Show that the solution vector x of Ax= b has only integer entries.Let A be a 3 × 3 upper triangular matrix with nonzero determinant. Show by explicit computation that A?1 is also upper triangular.