\(Bài1\)
\(a.\) \(a\cdot0=0;\) \(a:1=a;\) \(0:a=0\).
\(Bài5\)
\(1.50\cdot342\cdot2=\left(50\cdot2\right)\cdot342=100\cdot342=34200.\)
\(2.36\cdot97+97\cdot64=\left(36+64\right)\cdot97=100\cdot97=9700.\)
\(3.38\cdot63+37\cdot38=38\cdot\left(63+37\right)=38\cdot100=3800.\)
\(4.157\cdot289-289\cdot57=\left(157-57\right)\cdot289=100\cdot289=28900.\)
\(5.250\cdot1476\cdot4=\left(250\cdot4\right)\cdot1476=1000\cdot1476=1476000.\)
\(6.189\cdot509-189\cdot409=189\cdot\left(509-409\right)=189\cdot100=18900.\)
\(7.35\cdot34+35\cdot38+65\cdot15+65\cdot13=1190+1330+975+845\)
\(=4340.\)
\(8.37\cdot24+37\cdot76+63\cdot79+63\cdot21=37\cdot\left(24+76\right)+63\cdot\left(79+21\right)\)
\(=37\cdot100+63\cdot100=3700+6300=10000.\)
\(9.39\cdot8+60\cdot2+21\cdot8=39\cdot8+15\cdot4\cdot2+21\cdot8\)
\(=39\cdot8+15\cdot8+21\cdot8=\left(39+15+21\right)\cdot8=75\cdot8=600.\)
\(10.36\cdot28+36\cdot82+64\cdot69+64\cdot41\)
\(=\left(36\cdot28+36\cdot82\right)+\left(64\cdot69+64\cdot41\right)=36\cdot\left(28+82\right)+64\cdot\left(69+41\right)\)
\(=36\cdot110+64\cdot110=110\cdot\left(36+64\right)=110\cdot100=11000.\)
\(11.2\cdot17\cdot12+4\cdot6\cdot21+8\cdot3\cdot62\)
\(=\left(2\cdot12\right)\cdot17+\left(4\cdot6\right)\cdot21+\left(8\cdot3\right)\cdot62=24\cdot17+24\cdot21+24\cdot62\)
\(=24\cdot\left(17+21+62\right)=24\cdot100=2400.\)
\(12.20+21+22+...+29+39\)
\(=\left(20+39\right)+\left(21+29\right)+\left(22+28\right)+\left(23+27\right)+\left(24+26\right)+25\)
\(=59+50+50+50+50+25=284.\)
\(13.1+7+8+15+23+...+160\)
\(=1+7+8+15+23+38+61+99+160\)
\(=1+\left(8-1\right)+\left(15-7\right)+\left(23-8\right)+\left(38-15\right)+\left(61-23\right)+\left(99-38\right)+\left(160-61\right)+\left(259-99\right)\)
\(=-7+160+259=412.\)
\(14.1+4+5+9+14+...+60+97\)
\(=1+4+5+9+14+23+37+60+97\)
\(=5+5+23+23+97+97=250.\)
