题目内容

(
2
3
-
3
7
-
2
21
)×42+3
4
5
×7-1
4
5
×7．
分析：本题可运用乘法分配律进行简便计算．
解答：解：（
2
3
-
3
7
-
2
21
）×42+3
4
5
×7-1
4
5
×7，
=
2
3
×42-
3
7
×42-
2
21
×42+（3
4
5
-1
4
5
）×7，
=28-18-4+2×7，
=6+14，
=20．
点评：乘法分配律为：两个数的和乘一个数，可以先把它们分别与这个数相乘，再把结果合并起来．
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