Use Gauss–Jordan reduction to solve each of the following systems. (a) x1 + x2 =−1 4x1 −3x2 = 3 (b) x1 +3x2 + x3 + x4 = 3 2x1 −2x2 + x3 +2x4 = 8 3x1 + x2 +2x3 − x4 =−1 (c) x1 +x2 +x3 =0 x1 −x2 −x3 =0 (d) x1 + x2 +x3 + x4 =0 2x1 + x2 −x3 +3x4 =0 x1 −2x2 +x3 + x4 =0