Question #228109

find the basis and the dimension of the solution space W of the following system

X1 + 2X2 - 2X3 + 2X4 - X5 = 0

X1 + 2X2 - X3 + 3X4 - 2X5 = 0

2X1 + 4X2 - 7X3 +X4 - X5 = 0


1
Expert's answer
2021-08-23T17:04:58-0400

Let us find the basis and the dimension of the solution space WW of the following system


{X1+2X22X3+2X4X5=0X1+2X2X3+3X42X5=02X1+4X27X3+X4X5=0\begin{cases} X_1 + 2X_2 - 2X_3 + 2X_4 - X_5 = 0\\ X_1 + 2X_2 - X_3 + 3X_4 - 2X_5 = 0\\ 2X_1 + 4X_2 - 7X_3 +X_4 - X_5 = 0 \end{cases}


After substracting from the second equation the first equation, and after multiplying the first equation by 2-2 and adding to the third equation, we get the following equivalent system:


{X1+2X22X3+2X4X5=0X3+X4X5=03X33X4+X5=0\begin{cases} X_1 + 2X_2 - 2X_3 + 2X_4 - X_5 = 0\\ X_3 + X_4 - X_5 = 0\\ -3X_3 -3X_4 +X_5 = 0 \end{cases}


Let us multiply the second equation by 3 and add to the third equation:


{X1+2X22X3+2X4X5=0X3+X4X5=02X5=0\begin{cases} X_1 + 2X_2 - 2X_3 + 2X_4 - X_5 = 0\\ X_3 + X_4 - X_5 = 0\\ -2X_5 = 0 \end{cases}


It follows that


{X1=2X2+2X32X4X3=X4X5=0\begin{cases} X_1 =- 2X_2 +2X_3 -2X_4 \\ X_3 =- X_4\\ X_5 = 0 \end{cases}


and hence


{X1=2X24X4X3=X4X5=0\begin{cases} X_1 =- 2X_2 -4X_4 \\ X_3 =- X_4\\ X_5 = 0 \end{cases}


We conclude that


W={(X1,X2,X3,X4,X5)R5:X1=2X24X4, X3=X4, X5=0}={(2X24X4,X2,X4,X4,0)R5:X2,X4R}={X2(2,1,0,0,0)+X4(4,0,1,1,0)R5:X2,X4R}.W=\{(X_1,X_2,X_3,X_4,X_5)\in\R^5: X_1 =- 2X_2 -4X_4,\ X_3 =- X_4, \ X_5 = 0\}\\ =\{(- 2X_2 -4X_4,X_2,-X_4,X_4,0)\in\R^5: X_2, X_4 \in\R\}\\ =\{X_2(- 2,1,0,0,0)+X_4(-4,0,-1,1,0)\in\R^5: X_2, X_4 \in\R\}.


Therefore, {(2,1,0,0,0),(4,0,1,1,0)}\{(- 2,1,0,0,0),(-4,0,-1,1,0)\} is a basis for the solution space WW of the system, and hence dim W=2.dim\ W=2.



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