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# Trigonometry watch

1. f(x) = 2(2cosx - sinx)sinx

iii)Show that f(x) = Rcos(2x - lambda) - 1

2sin2x - cos2x - 1 is found from ii) by rearranging f(x) but now i'm stuck on how to show 2sin2x - cos2x - 1 = Rcos(2x - lambda) - 1
2. (Original post by znx)
f(x) = 2(2cosx - sinx)sinx

iii)Show that f(x) = Rcos(2x - lambda) - 1

2sin2x - cos2x - 1 is found from ii) by rearranging f(x) but now i'm stuck on how to show 2sin2x - cos2x - 1 = Rcos(2x - lambda) - 1
If you haven't come across R formulae before (C3, Edexcel) then expand the Rcos.. and see what you get.

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