1/2A=1/2(6/4+6/28+6/70+6/130+6/208)

       = 3/4+3/28+3/70+3/130+3/208

       = 1-1/4+1/4-1/7+..................-1/16

       =1-1/16

       =15/16 => A=15/8

a, 2009

b, -24

Lời giải:
$60-42:6+4\times 7=60-7+4\times 7=60+7\times (4-1)$

$=60+7\times 3=60+21=81$

a,  

\(\frac{14}{6}+\frac{1}{9}+\frac{19}{13}+\frac{17}{9}+\frac{7}{13}+\frac{4}{6}\)

\(=\left(\frac{14}{6}+\frac{4}{6}\right)+\left(\frac{1}{9}+\frac{17}{9}\right)+\left(\frac{19}{13}+\frac{7}{13}\right)\)

\(=\frac{18}{6}+\frac{18}{9}+\frac{26}{13}\)

  \(=3+2+2\)

\(=7\)

b,

\(\frac{995}{997}x\frac{990}{993}x\frac{997}{990}x\frac{993}{995}x\frac{97}{95}\)

\(=\frac{995x990x997x993x97}{997x993x990x995x95}\)
\(=\frac{97}{95}\)

(Cùng triệt tiêu 995 ; 990 ; 997 ; 993 )

a, \(4^7.3^4.9^6:6^{13}\)

\(=\left(2^{14}.3^4.3^{12}\right):\left(2^{13}.3^{13}\right)\)

\(=2^{14}:2^{13}.3^{16}:3^{13}\)

\(=2.3^3=54\)

b, \(2^3.3^2-5^{16}:25^7\)

\(=72-5^{16}:5^{14}\)

\(=72-5^2=47\)

4^7.3^4.9^6:6^13=4^7.3^4.(3^2)^6:6^13

=4^7.3^16:3^13.2^13

=(2^2)^7.3^16:3^13.2^13

=2^14.3^16:3^13.2^13

=2.3^3

=54

a) 174 x 78 - 174 x 68 

 = 174 x (78-68)

 = 174 x 10

 = 1740

b) 1/3 +1/6 + 4/3 + 2/6 +2/3 + 5/6

 = (1/3 + 2/3 + 4/3) + (1/6 + 2/6 + 5/6)

 = 7/3 + 4/3

 = 11/3 

làm nhanh để mình tick cho nhé

a) 174 x 78 - 174 x 68

 =174 x (78 - 68 )

 =174 x 10

 =1740

b) 1/3 + 1/6 + 4/3 + 2/6 + 2/3 + 5/6

 = (1/3 + 4/3) + (1/6 + 5/6) + (4/3 + 2/3)

 = 4/4            +   6/6           + 6/3

 = 1 + 1+2

 =4

= 3570

= 357x(5-1+6) = 357 x10 = 3570

3570(dễ vãi...)