## teacherman79 one year ago how do you show the set V of all 2 x 2 upper triangular matrices is a subspace of M22?

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1. satellite73

verify all the conditions for the definition of a subspace, which is really the point of this exercise, to write them down

2. satellite73

for example, the identity matrix has to be included in the subspace, and $\begin{pmatrix} 1& 0 \\ 0 & 1 \end{pmatrix}$ is upper triangular, so it is in the subspace

3. satellite73

you have to show it is closed under addition check that $\begin{pmatrix} a & b \\ c & 0 \end{pmatrix}+\begin{pmatrix} e& f \\ g & 0 \end{pmatrix}$ is upper triangular, which it is etc

4. satellite73

check that the inverse of an upper triangular two by two matrix is also an upper triangular two by two matrix in other words, write down all the conditions necessary for it to be a subspace, then check them one by one

5. JamesJ

Um, upper triangular 2x2 matrices are matrices of the form $\begin{pmatrix} a & b \\ 0 & c \end{pmatrix}$ So some modifications to be made above. Also, the zero vector in $$M_{2 \times 2}$$ is the zero matrix, not the identity matrix.

6. satellite73

thanks for keeping me honest

7. teacherman79

@satellite73 @JamesJ Thank you both...this site is cool...wish I knew about it 6 weeks ago...