Question

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Answer 1

see how once you distribute the sin into (t + 6pi)

you get

sin t - sin t - sin 6pi

see how the sin t cancel out?

so all you have is -sin 6pi

6 pi just means you go around the circle 3 times...and end up back where you started (no matter the direction)

so sin at 0, or 360 degrees is? 0

Answer 2

no problem...and hope you feel better! :)