X²-2√2X-3
=X²-2√2X+(√2)²-(√2)²-3
=(X-√2)²-(√5)²
=(X-√2+√5)(X-√2-√5)
或
X²-2√2-3
=X²-(2√2+3)²
=X²-[(√2)²+2√2+1²]
=X²-(√2+1)²
=(X+√2+1)(X-√2-1)