tan⁴x - sec⁴x = 1 - 2sec²x
tan²x = sec²x - 1
tan⁴x - sec⁴x = 1 - 2sec²x
(tan²x)² - sec⁴x = 1 - 2sec²x
(sec²x - 1)² - sec⁴x = 1 - 2sec²x
(sec²x)² - 2 * sec²x + 1 - sec⁴x = 1 - 2sec²x
sec⁴x - 2 sec²x + 1 - sec⁴x = 1 - 2sec²x
sec⁴x can be cancelled:
- 2 sec²x + 1 = 1 - 2sec²x
1 - 2sec²x = 1 - 2sec²x
