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Its easy. Just remember tan x =1/cot x
If we substitute tan A = 1/cot A in the left hand side, we get
(1-tan A)/(1+ tan A) = (1-1/cot A)/(1+1/cot A) = [(cot A-1)/cot A] /[(cot A + 1)/cot A]
cancelling out cot A from both numerator and denominator, we get
cot A-1 / cot A + 1 = right hand side. Hence proved.
Start with the left hand side, and show that it is equivalent to the right hand side:
`=(cotA-1)/(cotA+1) ` as required.
** Recognizing that the cot is the reciprocal of the tan, you can shorten this by multiplying the left side by `(cotA)/(cotA) ` to get the desired result immediately.**
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