Manipulate normally, noting:
- cot x = cos x / sin x
- cos² x + sin² x = 1 → sin²x = 1 - cos² x
- a² - b² = (a + b)(a - b)
- 1 = 1²
- ab = ba
- a/(bc) = a/b/c
(1 + cot x)² - 2 cot x = 1² + 2 cot x + cot² x - 2 cot x
= 1 + cot² x
= 1 + (cos x / sin x)²
= 1 + cos² x / sin² x
= 1 + cos² x / (1 - cos² x)
= ((1 - cos² x) + cos² x)/(1 - cos² x)
= 1/(1² - cos² x)
= 1/((1 + cos x)(1 - cos x))
= 1/(1 - cos x)/(1 + cos x)
QED.