已知a=12+3

∵a=
1
2+
3，
∴a=2-
3＜1，
∴原式=
(a−1)2
a−1-

(a−1)2
a(a−1)
=a-1-
1−a
a(a−1)
=a-1+
1
a
=2-
3-1+2+
3
=4-1
=3．