a) 81x^4 + 4 = 81x4 +4 = (9x² +2)² - 36x² = (9x² +2 -6x)( 9x² +2 +6x)
b) x4 + 2013x2 +2012x +2013
= x4 + 2013x² + 2013x - x+ 2013
= x4 - x + 2013x² + 2012x + 2013
= x(x^3-1) + 2013(x² +x+1)
= x(x-1)(x²+x+1) + 2013(x² +x+1)
= (x² +x+1)(x² - x +2013)
Chúc bạn học tốt ^^