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# [M2] Integration problem seeks for model answer steps

## Integration problem seeks for model answer steps

I have tried it for several time but I still can't find the model answer
Here are the question and working

Thank you very much

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 風之男 學院師父 發短消息 加為好友 當前離線 2# 大 中 小 發表於 2022-3-24 09:17 PM (第 55 天) 只看該作者 [顯示] [隱藏] Be careful when you do the actual integrate part. You're integrating over theta, not x, so all the x's in your last steps should be theta. How do you change back from theta to x? Well, x=3sin theta, so theta = arcsin x/3. Can you finish now? (It's quite a slog to go from sin4theta to x's using trig identities, but try it first.) UID216418 帖子970 精華0 積分118 閱讀權限40 在線時間1832 小時 註冊時間2011-5-4 最後登錄2022-5-14  查看詳細資料 TOP

## 回覆 2# 風之男 的帖子

Thank you very much for your sincere attention.
I can’t  modify sin 4 theta to the form of the answer.
Would you give me more hints?
I think I am not smart enough

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## 回覆 3# luckjoe 的帖子

I'm not too sure either, but there's an error in your working out; sin^2(2theta) = (1-cos4theta)/2 not (1+cos4theta)/2. So you should end up with a positive (sin4theta)/4 in your answer.

Also, just as a suggestion, if you rearrange your x/3 = sin(theta), you can get theta = arcsin(x/3).

But I'm not sure what to do with the sin4theta

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 風之男 學院師父 發短消息 加為好友 當前離線 5# 大 中 小 發表於 2022-3-25 07:16 PM (第 54 天) 只看該作者 Don't say you're not smart! You just don't know how to do it _yet_! So your goal is to express sin(4theta) in terms of sin(theta) and cos(theta). I'll start you off with sin(4theta) = 2sin(2theta)cos(2theta), can you continue? Now you might be asking why cos(theta)? Well, that's something you'd do quite a bit in trig. sub - draw a right triangle to find the "missing" length. There should be examples in your textbook. UID216418 帖子970 精華0 積分118 閱讀權限40 在線時間1832 小時 註冊時間2011-5-4 最後登錄2022-5-14  查看詳細資料 TOP

## 回覆 4# 妙妙貓 的帖子

Actually, expand the sin(4theta) using double angle formula, then use pythagoras' theorem to find cos(arcsinx/3) and manipulate it algebraically.

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## 回覆 5# 風之男 的帖子

Thank you very much for helping me to find the answer.
I’m so delighted that I have finally found the answer.

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## 回覆 6# 妙妙貓 的帖子

Thank you very much for your sincere helping

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