1. A = 2

2. x = 9

2,  tính vế trc 

\(\frac{1}{2}\left(\frac{2}{3.5}+\frac{2}{5.7}+......+\frac{2}{19.21}\right)=\frac{1}{2}\left(\frac{1}{3}-\frac{1}{5}+.....+\frac{1}{19}-\frac{1}{21}\right)=\frac{1}{2}\left(\frac{1}{3}-\frac{1}{21}\right)=\frac{1}{2}.\frac{2}{7}=\frac{1}{7}\)

=> 1/7 . x = 9/7

          x    = 9/7 . 7 

          x    = 9 

5) 327 - 48 - 152 + 173 =(327+173)-(48+152)=500-200=300

6) A = \(\frac{154+919+146+781}{823-217+533-139}=\frac{\left(154+146\right)+\left(919+781\right)}{1000}\)

=\(\frac{300+1700}{1000}=\frac{2000}{1000}=2\)

7) ( 1354 - 356 : 6 ) + 13,2=(1354\(-\frac{178}{3}\))+13,2=\(\frac{3884}{3}+13,2=\frac{19618}{15}\)

8) 784 . 0,5 : 0,28 - 196 : 28 + 2003=392:0,28-7+2003

=1400-7+2003=3396

Ta có: \(\left(2^{17}+15^4\right)\cdot\left(3^{19}-2^{17}\right)\cdot\left(2^4-4^2\right)\)

\(=\left(2^{17}+15^4\right)\left(3^{19}-2^{17}\right)\cdot\left(16-16\right)\)

=0

217-354+200-146-317

=(217-317)-(354+146)+200

=-100-500+200

=-600+200

=-400

217-354+200-146-317

=(217-317)-(354+146)+200

=-100-500+200

=-600+200

=-400

\(124 - (118 - x) = 217\)

\(118-x=124-217\)

\(118-x=-93\)

\(x=118-\left(-93\right)\)

\(x=211\)

\(---\)

\(315+(146-x)=401\)

\(146-x=401-315\)

\(146-x=86\)

\(x=146-86\)

\(x=60\)

\(---\)

\(575-(6x+70)=445\)

\(6x+70=575-445\)

\(6x+70=130\)

\(6x=130-70\)

\(6x=60\)

\(x=60:6\)

\(x=10\)

\(---\)

\(315+(125-x)=435\)

\(125-x=435-315\)

\(125-x=120\)

\(x=125-120\)

\(x=5\)

#\(Toru\)

 124 - ( 118 - x ) = 217 

   118 - x =124-217

118 - x =-93

x=118-(-93)

x=      211                                                                                    

315 + ( 146 - x ) = 401 \

146 - x=401-315

146 - x=     83

x=   146-83

x=63                                                                                     

575 - ( 6x + 70 ) = 445   

     6x + 70=575-445

 6x + 70=130

 6x=130-70

 6x =60

x=60:6

x=10                                                                                     

315 + ( 125 - x ) = 435

125 - x=435-315

124-x=120

x=124-120

x=4

124 + (118-x)=217

=> 118-x=217-124

=> 118-x=93

=> x=118-93

=> x=25

(x-47)-115=0

=> x-47=0+115

=> x-47=115

=> x=115+47

=> x=162

315+(146-x)=401

=> 146-x=401-315

=> 146-x=86

=> x=146-86

=> x=60

124+(118-x)=217 => 118-x=217-124=> 118-x=93=> x=118-93 => x=25

(x-47)-115=0 => x-47=0+115=>x-47=115=>x=115+47=>x=162

315+(146-x)=401=>146-x=401-315=>146-x=86=>x=146-86=>x=60