a2-2ab-b2/ a2-3ab+b2?
应该是(a²-2ab-b²)/ (a²-3ab+b²)吧?
已知a/b=1/3,b=3a,
则(a²-2ab-b²)/ (a²-3ab+b²)
=[a²-2a(3a)-(3a)²]/ [a²-3a(3a)+(3a)²]
=[a²-6a²-9a²]/ [a²-9a²+9a²]
=-14a²/a²
=-14