(1+tanx)/(1-tanx)=(1+cotx)/(1-cotx)

(1+tanx)/(1-tanx)=(1+sinx/cosx)/(1-sinx/cosx)

= ((cosx+sinx)/cosx))/((cosx-sinx)/cosx))

= (cosx+sinx)/(cosx-sinx)

We divide up and down by sinx to make cotx appear

= (cotx+1)/(cotx-1)

This is the right expression

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