Ý D đó bn.Chúc bn hok tốt nhé.

\(=\frac{3.\left(\frac{1}{51}-\frac{4}{57}+\frac{11}{59}\right)}{5.\left(\frac{1}{51}-\frac{4}{57}+\frac{11}{59}\right)}=\frac{3.1}{5.1}=\frac{3}{5}\)

ta có

A=1/20 + 1/21+1/22+....+1/59
   =(1/20+1/21+...+1/39)+(1/40+1//41+....+1/59)<1/20.20+1/40.20=1 + 1/2=3/2
vậy A<3/2
Chúc bạn học tốt nha ^-^

Ta có \(A=\left(\frac{1}{20}+\frac{1}{21}+\frac{1}{22}+...+\frac{1}{39}\right)+\left(\frac{1}{40}+\frac{1}{41}+...+\frac{1}{59}\right)\)

\(A< \left(\frac{1}{20}+\frac{1}{20}+\frac{1}{20}+...+\frac{1}{20}\right)+\left(\frac{1}{40}+\frac{1}{40}+...+\frac{1}{40}\right)\)

\(A< \frac{20}{20}+\frac{20}{40}\)

\(A< \frac{3}{2}\)

ko co de]

\(\frac{3}{5.7}+\frac{3}{7.9}+\frac{3}{9.11}+...+\frac{3}{59.61}\) 

\(=\)\(\frac{3}{2}\left(\frac{2}{5.7}+\frac{2}{7.9}+\frac{2}{9.11}+...+\frac{2}{59.61}\right)\)

\(=\)\(\frac{3}{2}\left(\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}+...+\frac{1}{59}-\frac{1}{61}\right)\)

\(=\)\(\frac{3}{2}\left(\frac{1}{5}-\frac{1}{61}\right)\)

\(=\)\(\frac{3}{2}.\frac{56}{305}\)

\(=\)\(\frac{84}{305}\)

Chúc bạn học tốt ~ 

\(\frac{19}{20}-\frac{5}{6.7}-\frac{5}{7.8}-\frac{5}{8.9}-...-\frac{5}{59.60}\)

= \(\frac{19}{20}-\left(\frac{5}{6.7}+\frac{5}{7.8}+\frac{5}{8.9}+...+\frac{5}{59.60}\right)\)

= \(\frac{19}{20}-5\left(\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+...+\frac{1}{59.60}\right)\)

= \(\frac{19}{20}-5\left(\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+...+\frac{1}{59}-\frac{1}{60}\right)\)

= \(\frac{19}{20}-5\left(\frac{1}{6}-\frac{1}{60}\right)\)

= \(\frac{19}{20}-5.\frac{3}{20}\)

= \(\frac{19}{20}-\frac{3}{4}\) = \(\frac{1}{5}\)

a)Ta đặt A=10+15+...+300

Số số hạng của A là:(300-10):5+1=59(số)

Tổng của A là:(10+300).59:2=9145

=>9145+x=6750

=>x=6750-9145

=>x=-2395

b)\(\frac{1}{42}+\frac{1}{30}+\frac{1}{20}+\frac{1}{12}+\frac{1}{6}+\frac{1}{2}-\frac{1}{x+1}=\frac{59}{77}\)

<=>\(\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{6.7}-\frac{1}{x+1}=\frac{59}{77}\)

<=>\(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{6}-\frac{1}{7}-\frac{1}{x+1}=\frac{59}{77}\)

<=>\(1-\frac{1}{7}-\frac{1}{x+1}=\frac{59}{77}\)

<=>\(\frac{6}{7}-\frac{1}{x+1}=\frac{56}{77}\)

<=>\(\frac{1}{x+1}=\frac{6}{7}-\frac{56}{77}=\frac{66}{77}-\frac{56}{77}\)

<=>\(\frac{1}{x+1}=\frac{10}{77}\)

<=>10(x+1)=77

<=>10x+10=77

<=>10x=67

<=>x=6,7

bai 1

20:(24-(3^2.4-34)^2)-20

=20:(24-(9.4-34)^2)-20

=20:(24-2^2)-20

=20:20-20

=-19

bai 2

a

14+5x=4^6:4^3

14+5x=4^3

14+5x=64

5x=64-14

5x=50

x=50:5

x=10

b

189-2.(93-x)=59

2..(93-x)=189-59

2.(93-x)=130

(93-x)=130:2

93-x=65

x=93-65

x=28

1) \(20:\left[24-\left(3^2.4-34\right)^2\right]-20\)

\(=20:\left[24-\left(9.4-34\right)^2\right]-20\)

\(=20:\left[24-4\right]-20\)

\(=20:20-20\)

\(=1-20\)

\(=-19\)

2)a.\(14+5x=4^6:4^3\)

       \(14+5x=64\)

       \(5x=64-14\)

       \(5x=50\)

    \(\Rightarrow x=10\)

b)     189-2(93-x)=59

         2(93-x)       =189-59

        2(93-x)        =130

           93-x          =130:2

           93-x          =65

               x            =93-65

               x            =28

Học tốt nha!!!

\(159\left(18-59\right)-59\left(19-159\right)\)

\(=159.\left(-41\right)-59.\left(-140\right)\)

\(=-6519-\left(-8260\right)\)

\(=1741\)

\(159.\left(15-59\right)-59.\left(19-159\right)\)

\(=159.15-159.59-59.19+59.159\)

\(=159.15-59.19=1264\)