Question

20．在□里填上適當(dāng)?shù)臄?shù)，使式子成立．
（1）□÷1.25+3.4=4.2
（2）（40-0.72×□）+425÷17=47
（3）0.84÷1.2×（10-□）-1.89=3.5
（4）7.28×[□-8.7÷（2.07-1.2）]=0．

Answer 1

分析 根據(jù)題意，把□中的數(shù)，看作未知數(shù)，然后再根據(jù)等式的性質(zhì)進(jìn)行解答．

解答 解：（1）□÷1.25+3.4=4.2
□÷1.25+3.4-3.4=4.2-3.4
□÷1.25=0.8
□÷1.25×1.25=0.8×1.25
□=1；

（2）（40-0.72×□）+425÷17=47
                （40-0.72×□）+25=47
            （40-0.72×□）+25-25=47-25
                             40-0.72×□=22
                 40-0.72×□+0.72×□=22+0.72×□
                             22+0.72×□=40
                        22+0.72×□-22=40-22
                                  0.72×□=18
                          0.72×□÷0.72=18÷0.72
□=25；

（3）0.84÷1.2×（10-□）-1.89=3.5
                 0.7×（10-□）-1.89=3.5
         0.7×（10-□）-1.89+1.89=3.5+1.89
                         0.7×（10-□）=5.39
                   0.7×（10-□）÷0.7=5.39÷0.7
                                       10-□=7.7
                                   10-□+□=7.7+□
                                      7.7+□=10
                                7.7+□-7.7=10-7.7
□=2.3；

（4）7.28×[□-8.7÷（2.07-1.2）]=0
                     7.28×[□-8.7÷0.87]=0
                              7.28×[□-10]=0
                      7.28×[□-10]÷7.28=0÷7.28
□-10=0
□-10+10=0+10
□=10．

點(diǎn)評 此題利用了解方程的思想進(jìn)行解決，簡潔易行．