<PRE><B>Question 550088</B>
1 2 3 4
6 7 8 9
-1 -2 -3 -4
-6 -7 -8 -9 
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calculate the cofactor of the element 9 of this determinant: 
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If you mean where the 9 is, element 2,4:
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= Det of
1 2 3
-1 -2 -3
-6 -7 -8 
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= 1(16-21) - 2(8-18) + 3(7-12) = -5 + 20 - 15
= zero
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