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Math in Focus Trig Qs (1 Viewer)

a1079atw

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Hi guys,

I've literally spent one hour on this type of question (acosx+bsinx = Rcos(x−α)) and still can't figure out the right answer...
Screen Shot 2013-11-01 at 4.49.46 pm.jpg
May anyone please help me? I don't know where I did wrong. Here's my solution (didn't finish as I knew it's know the right answer)
2013-11-01 16.55.03.jpg
Answer:
Screen Shot 2013-11-01 at 4.59.33 pm.jpg
 

QZP

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Hi, I had a quick inspection of your working out and I believe you dealt with the angles wrong. Also, you should put at the start of your solution "let ... = Rcos(x-a)" and then equate coefficients etc to find R and a otherwise it makes no sense :S
 

funnytomato

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Hi, I had a quick inspection of your working out and I believe you dealt with the angles wrong. Also, you should put at the start of your solution "let ... = Rcos(x-a)" and then equate coefficients etc to find R and a otherwise it makes no sense :S
+1

also note cos(a+b)=cos(a)*cos(b)-sin(a)*sin(b)
and cos(a-b)=cos(a)*cos(b)+sin(a)*sin(b)
 

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