Đáp án:

$\displaystyle \begin{array}{{>{\displaystyle}l}} Bài\ 3:\\ a.\ 1400;\ b.\ 30000;\ c.\ 126000\\ d.\ \ 17000;\ e.\ 3700;\ f.\ 423423\\ g.\ 33932;\ h.\ 6767;\ i.\ 6633\\ Bài\ 4:\\ a.\ 2400;\ b.\ 240;\ c.\ 3700;\ d.\ 12000;\ e.\ 810\\ Bài\ 5:\\ a.\ x=12;\ b.\ x=26;\ c.\ 43;\ d.\ x=7\ hoặc\ x=4\\ e.\ x=69;\ f.\ x=12;\ g.\ x=11 \end{array}$

Giải thích các bước giải:

$\displaystyle \begin{array}{{>{\displaystyle}l}} Bài\ 3:\\ a.\ 8.7.25=( 8.25) .7=200.7=1400\\ b.\ 48.125.5=6000.5=30000\\ c.\ 125.1008=125.( 1000+8) =125.1000+125.8\\ =125000+1000=126000\\ d.\ 8.17.125=( 8.125) .17=1000.17=17000\\ e.\ 4.37.25=( 4.25) .37=100.37=3700\\ f.\ 423.1001=423.( 1000+1) =423.1000+423=423423\\ g.\ 998.34=( 1000-2) .34=34.1000-34.2=33932\\ h.\ 67.101=67.( 100+1) =67.100+67=6767\\ i.\ 67.99=67.( 100-1) =67.100-67=6633\\ Bài\ 4:\\ a.\ 24.57+43.24=24.( 57+43) =24.100=2400\\ b.\ 12.19+12=12.( 19+1) =12.20=240\\ c.\ 37.38+62.37=37.( 38+62) =37.100=3700\\ d.\ 43.27+93.43+57.61+59.57\\ =43.( 27+93) +57.( 61+59) =43.120+57.120\\ =120.( 43+57) =120.100=12000\\ e.\ 64.6+81.4+17.6=6.( 64+17) +81.4\\ =6.81+81.4=81.( 6+4) =81.10=810\\ Bài\ 5:\\ a.\ ( x-12) .105=0\Leftrightarrow x-12=0\Leftrightarrow x=12\\ b.\ 47.( 27-x) =47\Leftrightarrow 27-x=1\Leftrightarrow x=26\\ c.\ ( x-40) .15=15.3\Leftrightarrow x-40=3\Leftrightarrow x=43\\ d.\ ( x-7)( 2x-8) =0\\ \Leftrightarrow x-7=0\ hoặc\ 2x-8=0\\ \Leftrightarrow x=7\ hoặc\ x=4\\ e.\ 2x+69.2=69.4\Leftrightarrow 2x=69.4-69.2=69.2\Leftrightarrow x=69\\ f.\ 2x-12-x=0\Leftrightarrow x-12=0\Leftrightarrow x=12\\ g.\ ( x+1) +( x+2) +...+( x+10) =165\\ \Leftrightarrow 10.x+( 1+2+...+10) =165\\ \Leftrightarrow 10x+( 10+1) .\left(\frac{10-1}{1}\right) :2=165\\ \Leftrightarrow 10x=165-55=110\\ \Leftrightarrow x=11 \end{array}$