Question #144525
Let
W = ((X1, X2, X3) belongs to R³ : X2 + X3 = 0).
Show that W is a subspace of R³ . Find two
subspaces W1 and W2 of R3 such that
R³ = W (direct sum )W1 and R³ = W (direct sum) W2 but W1 (not equal to) W2.
1
Expert's answer
2020-11-18T19:27:07-0500

Given

W={(x1,x2,x3)R3:x2+x3=0}W=\{(x_1,x_2,x_3)\R^3:x_2+x_3=0\}

Let a,bR&(x1,x2,x3),(y1,y2,y3)R3a,b\in \R\&(x_1,x_2,x_3),(y_1,y_2,y_3)\in\R^3

Now,


a(x1,x2,x3)+b(y1,y2,y3)=(ax1+by1,ax2+by2,ax3+by3)a(x_1,x_2,x_3)+b(y_1,y_2,y_3)=(ax_1+by_1,ax_2+by_2,ax_3+by_3)

Note that

ax2+by2+ax3+by3=a(x2+x3)+b(y2+y3)=0    (ax1+by1,ax2+by2,ax3+by3)Wax_2+by_2+ax_3+by_3=a(x_2+x_3)+b(y_2+y_3)=0\\ \implies (ax_1+by_1,ax_2+by_2,ax_3+by_3)\in W

Hence, W is a subspace of R3\R^3 .


Let us define a subspace

W1={(x2,x2,0)}W2={(x2,2x2,0)}W_1=\{(x_2,x_2,0)\}\\ W_2=\{(x_2,2x_2,0)\}

Clearly,


()W1W2={0},W1W={0},W2W={0}(\star)\,W_1\cap W_2=\{0\},W_1\cap W=\{0\},W_2\cap W=\{0\}

And,


R3=WW1\R^3=W\oplus W_1\\

As, for any vR3\vec v\in \R^3 we can write v=u+w\vec v=\vec u+\vec w for some uW,wW1\vec u\in W,\vec w\in W_1 and hold ()(\star) .


Similarly for

R3=WW2\R^3=W\oplus W_2


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