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How do you find the value of ##tan45## using the half angle identity?
Using ##tan 90^o=+-oo##, tan 45^o=1
##tan 90^o=+-oo=(2 tan 45^o)/(1-tan^2 45^o##.
Equating the denominator to 0,
##tan^2 45^o=1 and tan 45^o=+-1##
The first quadrant tangent ##tan 45^o > 0##.
So, ##tan 45^o = 1##..
Note that tan x is discontinuous at x = ##90^## and that
limits ##tan 90+-^o=+-oo##.