a) $a + 3 = -(-a - 3) = -(\sqrt{-a} + \sqrt 3)(\sqrt{-a} + \sqrt 3)$ (Vì a < 0 nên -a > 0)
b) $4a + 1 = -(-4a -1) $
$= -(\sqrt{-4a} + \sqrt 1)(\sqrt{-4a} - \sqrt 1 )$ (Vì a < 0 nên -4a > 0)
$=-(2\sqrt{-a} + \sqrt 1)(2\sqrt{-a} - \sqrt 1 )$
c) $2a+3$
$=-(-2a -3)$
$=-(\sqrt{-2a} + \sqrt 3)(\sqrt{-2a} + \sqrt 3)$ (Vì a < 0 nên -2a > 0)