口算30×40时，先用3乘4得 ，再在得数末尾添 个0，结果是 ．
直接写得数.+=+= 1﹣=﹣=﹣=﹣=++1=++=+=+= 1﹣﹣=+﹣=
口算.+=﹣=+﹣= 3+=+= 1﹣﹣=++= 4﹣=
计算：．
= ．
+
=
+
= 1﹣
=
﹣
=
﹣
﹣
=
+
+1=
+
+
=
+
=
+
= 1﹣
﹣
=
+
﹣
=
+
﹣
=
+
﹣
= 3+
=
+
= 1﹣
﹣
=
+
+
= 4﹣
=
．
= ．
= ．
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