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Reply 20
Yay i did it! :P Ahh i wish there was a way to spot which equation to use, i did a, but now b is just as challenging, and every one will be, how can people do these in less then 15 mins i don't know! Unless they guess correctly first time! AHHH
Reply 21
You just need to practice them to be able to spot them quickly, really.

If you're still having trouble with cosθ=sinθ2cos\theta = sin\frac{\theta}{2}

Consider cosθ=cos2θ2sin2θ2cos\theta = cos^2\frac{\theta}{2} - sin^2\frac{\theta}{2}

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