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tan 4x = 4tan x (1 – tan² x) / (1 – 6tan² x + tan⁴ x)
cos 4x = 1 – 8sin² x cos² x
Find the value of tan π/8.
Prove that cos² x + cos² (x + π/3) + cos² (x – π/3) = 3/2.
tan x = – 4/3, x in quadrant II
cos x = – 1/3, x in quadrant III
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