i never learned those formulas.. or maybe we're not there yet?? i don't know.
(cot^2 x -1)/(cot^2 x +1)= 1-2sin^2 x
[(cos^2 x/ sin^2 x) -1] / [(cos^2 x/sin^2 x) +1]
[(cos^2x*sin^2 x/sin^4 x) -1]/[(cos^2x*sin^2 x/sin^4 x) +1]
then.. i don't know. i'll just ask my teacher, maybe she has some way to do it using methods i have already learned.