(tg x + ctg x)² - (tg x -ctg x)² = 4
(tg x + ctg x)² - (tg x -ctg x)² =(tg x + ctg x -(tg x -ctg x))*(tg x + ctg x +(tg x -ctg x))= =2 *ctg x * 2* tg x=4