Find the value of the determinant:
where A, B, C are the angles of a triangle.
EDIT: I corrected the typo
Hello,
I think there's a typo in red. (which red_dog considered...)
Sarrus' rule gives the value :
and recall these values : ,
-->
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Another approach.
Multiply the first row by , the second row by , the third row by .
This gives (implicitly using ) :