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by **testing** on Mon Feb 09, 2009 12:57 am

Find two 2-by-2 invertible matrices whose sum is not invertible.

Do I just start making up matrices, until I find something that works?

**Sponsor**

by **stapel_eliz** on Mon Feb 09, 2009 1:23 pm

testing wrote:Do I just start making up matrices, until I find something that works?

Yes and no. Random matrices may not be helpful. Instead, think about the relationship between invertible matrices and their determinants, and see if you can think of a way to use this to create a useful pair. :wink:

Eliz.

by **testing** on Mon Feb 09, 2009 9:19 pm

The determinant for a non-invertible matrix is zero, so these should work:

$\left[\begin{array}{rr}1&0\\0&1\end{array}\right]$

$\left[\begin{array}{rr}-1&0\\0&-1\end{array}\right]$

If you add them, you get:

$\left[\begin{array}{rr}0&0\\0&0\end{array}\right]$

And any two matrices like that will work, so that's loads of answers right there. :wink:

by **stapel_eliz** on Tue Feb 10, 2009 12:03 am

Exactly!

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Find two 2-by-2 invertible matrices with sum not invertible

Find two 2-by-2 invertible matrices with sum not invertible

Re: Find two 2-by-2 invertible matrices with sum not invertible