How to factorise (x^4 - 1) to (x^2+1)(x+1)(x-1)?

Use a^2-b^2=(a+b)(a-b) two times

(x^4 - 1)
=(x^2+1)(x^2-1) [first time]
=(x^2+1)(x+1)(x-1) [second time]

首先,利用A^2-B^2=(A+B)(A-B)既公式:
(x^2+1) (x^2-1)
注意:只可以-數先用得以上既公式, 所以(x^2+1)保留~
(x^2+1) (x+1)(x-1)
OK.