# Physics 1251 HW 8 #9

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Here is the question:

To get this answer, you need to use "Emf =  - dΦB/dt = - d/dt (B*A*N)*sin(theta)"

My question is, why don't you use cos(theta) since you are using the formula for magnetic flux...looking at your slides, you say:

How do I know when you use sin/cos in a problem like this?