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helo me solve a question about trigonometry please

I still got the wrong answer for b after tried several times. anyone can help me? I know how to do the other ones by the way... IMG_20200328_212815-compressed.jpg.jpeg
Reply 1
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nzy
17
Have you tried rewriting the left hand side of the equation in the form Rcos(2theta + a), where R and a are the values found in part a), and then solve for theta as you would normally?
Original post by nzy
Have you tried rewriting the left hand side of the equation in the form Rcos(2theta + a), where R and a are the values found in part a), and then solve for theta as you would normally?


IMG_20200329_001816-compressed.jpg.jpegthis is what I did. and I got only one answer. which is wrong. is that due to the erroneous interval or what? thank u
Reply 3
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nzy
17
(Sorry for the late reply).
You substituted the LH side of the equation incorrectly. 13cos(2theta + 22.62) = -6.5, hence why one of the solutions is half of your answer.

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