x-5=13-x

x-5-13+x=0

2x-18=0

         2x=18

           x=18:2=9

c=4

b=0

a=2

c = 4

b = 0

a = 2

Đề đâu bạn

S= - 3²\(\left(\frac{1}{4}+\frac{1}{28}+\frac{1}{70}+...+\frac{1}{868}\right)\)

S = - 3²\(\left(\frac{1}{1.4}+\frac{1}{4.7}+\frac{1}{7.10}+...+\frac{1}{28.31}\right)\)

S = - 3\(\left(\frac{3}{1.4}+\frac{3}{4.7}+...+\frac{3}{28.31}\right)\)

S = -3\(\left(1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+...+\frac{1}{28}-\frac{1}{31}\right)\)

S = -3 \(\left(1-\frac{1}{31}\right)\)

S = -3\(.\frac{30}{31}\)

S = -90/31

1/3S=-(1/1*4+1/4*7+1/7*10+...+1/28*31)=-(1/1-1/4+1/4-1/7+1/7-1/10+...+1/28-1/31)=-(1/1-1/31)=-30/31

=>S=(-30/31):1/3=-90/31

4.2^x - 3 = 123

  = > 123 + 3 = 4.2^x

= > 126 = 4.2^x

= 126 = 8^x

= > x = 126 : 8 

x = 15,75 (16)

Đs:

Ps: Hình như đề bài có vấn đề, còn nếu mình làm sai thì đừng trách nhá

(=) 4.2\(^x\)=123+3

(=) 4.2\(^x\)=126

(=) 2\(^x\)=126/4

(=) 2\(^x\)= 31.5

=> hình như sai đề, ktra lại đề r giải giống như trên

hãy giúp mìnhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh

ko hiểu

\(A=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+.....+\frac{1}{99}-\frac{1}{100}.\)

\(A=\frac{1}{2}-\frac{1}{100}=\frac{100}{200}-\frac{2}{200}=\frac{98}{200}\)

\(A=\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{99.100}\)

\(A=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-...+\frac{1}{99}-\frac{1}{100}\)

\(A=\frac{1}{2}-\frac{1}{100}\)

\(A=\frac{50}{100}-\frac{1}{100}=\frac{50-1}{100}=\frac{49}{100}\)

\(2345-1000:\left(19-2\left(21-18\right)^2\right)\)

\(2345-1000:\left(19-2.3^2\right)\)

\(2345-1000:\left(19-2.9\right)\)

\(2345-1000:\left(19-18\right)\)

\(2345-1000:1\)

\(2345-1000\)

\(1345\)

2345 - 1000 : [ 19 - 2 ( 21 - 18 )² ]

= 2345 - 1000 : ( 19 - 2.3² )

= 2345 - 1000 : ( 19 - 2.9 )

= 2345 - 1000 : ( 19 - 18 )

= 2345 - 1000 : 1

= 2345 - 1000

= 1345

50-4.(x-20)= 4.8

50-4x+80=32

130-4x=32

-4x=32-130

-4x=-98

x=(-98):(-4)

x=49/2

50 - 4(x - 20) = 4.8

50 - 4(x - 20) = 32

       4(x - 20) = 50 - 32

       4(x - 20) = 18

         (x- 20) = 18 : 4

         x- 20 = 4,5

         x       = 4,5 + 20

        x         = 24,5