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# Need help with this trigonometric question involving r cos(2x + a). Confused Watch

1. (Original post by bryan966)
Hey there. This is what my mom's colleague got. I'm a bit confused whether is a valid solution. A lil warning tho, it takes time to know which part comes first. Cheers
OK, squaring and rearranging so everything contains either cos^2 x or sinxcosx will let you use a cos(2x + a) type expansion on it and solve. I can't believe this would be taught as a practical technique though - you're making the problem more difficult just to go back and solve it in a different way!

Also, when you square an equation you have to make sure you don't generate any spurious solutions e.g. if you start with A = 4 then A^2 = 16, but the second equation would also be solved by A = -4.
2. there s wrong working at blue line. the expansion din complete
3. The question in that form just seems kind of pointless. The point of this technique is to allow you to isolate x by writing the original equation in terms of one trig function which then can be inverted.

This Q is like saying here is a question with a standard solution technique but we want you to do something obscure and pointless.

Hence, it is almost certainly a misprint or typo of some kind.

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Updated: May 20, 2016
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