... du.o.ng v´o.ihˆe.x1+2x2− 3x3+4x 4 =7,2x1+4x2+5x3− x 4 =2hay2x2− 3x3=7− x1− 4x 4 ,4x2+5x3=2− 2x1+ x 4 . 148 Chu.o.ng 4. Hˆe.phu.o.ng tr`ınh tuyˆe´n ... 3x2− 2x 4 3, x3=1 − 2x 4 3, x2,x 4 t`uy ´y)10.x1+2x2+3x3+4x 4 =5,x2+2x3+3x 4 =1,x1+3x3+4x 4 =2,x1+ x2+5x3+6x 4 =1.(DS. x1=15 4 , x2=32, ... x3= −13 4 , x 4 =2)11.x1+2x2+3x3+4x 4 =30,−x1+2x2− 3x3+4x 4 =10,x2− x3+ x 4 =3,x1+ x2+ x3+ x 4 =10.(DS. x1=1,x2=2,x3=3,x 4 =4) 12.5x1+...