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# Roots and trig

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1. Not sure how to approach C. Tried saying that cos(3x) is root 2 but that's not valid; so there can't be 3 roots, I know that much from that. Maybe expressing root 2 in terms of cos(15) or sin(15)? But I'm unsure where to proceed from that.

a) Got

b) I said

and after using compound angle formulae on the right side, I got

so is a root.

Other two roots are; .
2. (Original post by RDKGames)

Not sure how to approach C. Tried saying that cos(3x) is root 2 but that's not valid; so there can't be 3 roots, I know that much from that. Maybe expressing root 2 in terms of cos(15) or sin(15)? But I'm unsure where to proceed from that.

a) Got

b) I said

and after using compound angle formulae on the right side, I got

so is a root.

Other two roots are; .
start by letting 2x=y and then dividing through by 2
3. (Original post by RDKGames)
...

Note that so the above is...
4. (Original post by ValerieKR)
start by letting 2x=y and then dividing through by 2
Okay that one caught me out completely. Didn't even notice the coefficients weren't the same :/
Thanks, I'll give this a go now.

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