Prove the identity.
tan^5x = tan x (sec² x - 2 sec² x + 1)
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goal:tan^5x=tanx(sec²x-2sec²x+1)
proof: from l.h.s to r.h.s
tan^5x=(tanx)^5=((tanx)(tan²x)(tan²x))
from trigonometry identity
(tan²x)=sec²x-1
tan^5x=(tanx(sec²x-1)(sec²x-1))
=(tanx(sec^4x-sec²x-sec²x+1))
=(tanx(sec^4x-2sec²x+1))
proved