由题意可知:a-c-R=m,a+c-R=n,
则a-c=m+R,a+c=n+R,
故2b=2a
a 2 - c 2 =2
(a-c)(a+c)
=2
(m+R)(n+R) .
故答案为2
(m+R)(n+R)