mistake 
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Tareq Musa
· February 16, 2015
in video tutorials algebra basic 20th video at 3:13 min.

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Arnold Janssen
· March 7, 2015
I would be very glad if you could help me understand and solve these questions.
1) Suppose E1;E2;E3 are 4 by 4 identity matrices, except E1 has a;b;c in column 1 and E2 has d;e in column 2 and E3 has f in column 3 (below the 1’s). Multiply L =E1.E2.E3 to show that all these nonzeros are copied into L. E1E2E3 is in the opposite order from elimination (because E3 is acting ?rst). But E1E2E3 =L is in the correct order to invert elimination and recover A. 
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Alex Micharski
· February 22, 2015
The instructor probably thinks 9+10=21.

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