Đáp án:
a, 21 + 2x - 8x²
= -8x² + 14x - 12x + 21
= -2x(4x - 7) - 3(4x - 7)
=(4x - 7)(-2x - 3)
b, 6x² - 11x -10
=6x² - 15x + 4x - 10
=3x(2x - 5)+2(2x - 5)
=(2x - 5)(3x + 2)
c, x³ - 5x² + 8x - 4
=x³ - x² - 4x² + 4x + 4x -4
=x²(x - 1) - 4x(x - 1) + 4(x - 1)
=(x - 1)(x² - 4x + 4)
=(x - 1)(x - 2)²
d, 4x² - 8x - 5
= 4x² + 2x - 10x - 5
=2x(2x + 1) - 5(2x + 1)
=(2x + 1)(2x - 5)
e, x² + 2(a + b)x + 4ab
=x² + 2ax + 2bx + 4ab
=x(x + 2a) + 2b( x + 2a)
=(x + 2a)(x + 2b)
f, 4x² - 4x - 3
=4x² + 2x - 6x - 3
=2x(2x + 1) - 3(2x + 1)
=(2x + 1)(2x -3)