解:(1)49
![](http://thumb.zyjl.cn/pic5/latex/438501.png)
,
=(50-
![](http://thumb.zyjl.cn/pic5/latex/5139.png)
)×(-5),
=-249
![](http://thumb.zyjl.cn/pic5/latex/602.png)
;
(2)(-8)×(-7.2)×(-2.5)×
![](http://thumb.zyjl.cn/pic5/latex/927.png)
,
=-(8×
![](http://thumb.zyjl.cn/pic5/latex/438503.png)
),
=-60;
(3)-7.8×(-8.1)×0×|-19.6|=0;
(4)-|-0.25|×(-5)×4×(-
![](http://thumb.zyjl.cn/pic5/latex/5139.png)
),
=-0.25×(-5)×4×(-
![](http://thumb.zyjl.cn/pic5/latex/5139.png)
),
=-
![](http://thumb.zyjl.cn/pic5/latex/36.png)
.
分析:根據(jù)有理數(shù)的乘法法則:兩數(shù)相乘,同號(hào)為正,異號(hào)為負(fù),并把絕對(duì)值相乘.幾個(gè)不等于零的數(shù)相乘,積的符號(hào)由負(fù)因數(shù)的個(gè)數(shù)決定:當(dāng)負(fù)因數(shù)有奇數(shù)個(gè)數(shù),積為負(fù);當(dāng)負(fù)因數(shù)的個(gè)數(shù)為偶數(shù)個(gè)時(shí),積為正.任何數(shù)字同0相乘,都得0.
點(diǎn)評(píng):本題考查了有理數(shù)的乘法,兩數(shù)相乘,同號(hào)為正,異號(hào)為負(fù),并把絕對(duì)值相乘.幾個(gè)不等于零的數(shù)相乘,積的符號(hào)由負(fù)因數(shù)的個(gè)數(shù)決定:當(dāng)負(fù)因數(shù)有奇數(shù)個(gè)數(shù),積為負(fù);當(dāng)負(fù)因數(shù)的個(gè)數(shù)為偶數(shù)個(gè)時(shí),積為正.任何數(shù)字同0相乘,都得0.