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Prove the identity $$\csc2\theta=\frac{\sec\theta\csc\theta}{2}$$

I've started by using a double angle identity, but I'm not sure how to continue from here or if this is right approach.

$$\csc2\theta=\frac{1}{\sin2\theta}=\frac{1}{2\sin\theta\cos\theta}$$

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  • $\begingroup$ Your left-hand side should be $\csc (2\theta)$ $\endgroup$
    – paw88789
    Commented Dec 4, 2018 at 19:20

4 Answers 4

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What you did is correct. Now, you do:$$\frac1{2\sin\theta\cos\theta}=\frac{\frac1{\cos\theta}\times\frac1{\sin\theta}}2=\frac{\sec\theta\csc\theta}2.$$

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Your approach is right, you just need to do the last step to get the desired result: What are $\frac{1}{\sin(\theta)}$ and $\frac{1}{\cos(\theta)}$?

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The thing you did is OK. The question which occurs is do you know how to prove $$\sin (2x) = 2\sin x \cos x$$

(Hint: use adition theorem for function $\sin $)

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You are already on the right track. Here is another approach.
GO! $$\begin{aligned}\csc{2\theta}&=\frac{1}{\sin{2\theta}}\\&=\frac{1}{2\sin\theta\cos\theta}\\&=\frac12\csc\theta\sec\theta\\&=\frac{\csc\theta\sec\theta}{2}\end{aligned}$$ I hope this helps.

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