a-b=6,ab=5
(a+b)²=a²+2ab+b²
=a²-2ab+b²+4ab
=(a-b)²＋4ab
=6²＋4×5
=36+20
=56
即
a+b=±√56=±2√14
所以
a²-b²=(a-b)(a+b)=6×（±2√14）=±12√14