.5) Let a k-form w be closed if dw = 0. Let a form w be exact if there exists a form 7 with w= dn. a) Show that every exact form is closed. b) Show that a l-form f1(x)da 1 + f2(x)dx2+f3(x)daz in R* is closed iff Dif; = D, f. for all i, j,