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Homework Statement
Find the determinant of the matrix {{cos 25°, sin° 65}, {sin 120°, cos 390°}} (sorry, can't latex). {cos 25°, sin° 65} is first row and {sin 120°, cos 390°} is the second one.
Homework Equations
cos(a + b) = (cos a)(cos b) - (sin a) (sin b)
The Attempt at a Solution
I know you can just plug the values in a calculator, but apparently you can solve it by using some trig identities. I also though about using some property of rotation matrices but couldn't find any that fit the problem. Anyway, this is as far as I got:
cos (a + b) = (cos 25°)(cos 390°) - (sin 65°) (sin 120°)