解: f = (x1+2x2-x3)^2+x2^2+5x3^2+2x2x3 = (x1+2x2-x3)^2+(x2+x3)^2+4x3^2 = y1^2 + y2^2 + 4y3^2.方法: 先集中消灭所有含x1的项, 然后是x2,......
