先化简,再求值:(a+12)
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先化简,再求值:(a+
)
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![先化简,再求值:(a+12)](/uploads/image/z/17722001-65-1.jpg?t=%E5%85%88%E5%8C%96%E7%AE%80%EF%BC%8C%E5%86%8D%E6%B1%82%E5%80%BC%EF%BC%9A%28a%2B12%29)
原式=(a+
1
2+a-
1
2)(a+
1
2-a+
1
2)+
1
a+1+
1
a−1,
=2a+
a−1+a+1
a2−1,
=
2a3−2a+2a
a2−1=
2a3
a2−1,
当a=
3时,
原式=
2(
3)3
(
3)2−1=3
3.
1
2+a-
1
2)(a+
1
2-a+
1
2)+
1
a+1+
1
a−1,
=2a+
a−1+a+1
a2−1,
=
2a3−2a+2a
a2−1=
2a3
a2−1,
当a=
3时,
原式=
2(
3)3
(
3)2−1=3
3.