The Student Room Group
Reply 1
Use the identities for cos(a+b) and cos(a-b).
Reply 2
Lawbutwhere?
Given that cos(x+30) = 3 cos (x-30), prove that tan x = - (3^1/2) / 2

cos(x+30) = 3cos(x-30)
cosxcos30 - sinxsin30 = 3(cosxcos30 + sinxsin30
root3/2 cosx - sinx/2 = 3root3/2 cosx + 3sinx/2
-2root3/2cosx = 2sinx
-root3/2 cosx = sinx
tanx = -root3/2
This type of question is quite standard in C3, there's tons of it.

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