a: =>21x=42

hay x=2

a: =>21x=42

hay x=2

S = 1/3 + 1/9 +1/27 + 1/135 + 1/405

S = 135/405 + 45/405 + 15/405 + 3/405 + 1/405

S = \(\frac{135+45+15+3+1}{405}\)

S = \(\frac{199}{405}\)

199/405                        

Bài 2: 

a: =>21x=42

hay x=2

ta có : 1/5+1/15=1-1/15=14/15

vậy 1/5+1/15+1/45+1/135+1/405=1-1/405=404/405

404/405 NHỚ **** ĐÓ

\(\frac{121}{405}\)

ta có :1/5 + 1/15 = 1- 1/15 =14/15 

vậy :1/5 + 1/15 + 1/45 + 1/135 + 1/405 = 1- 1/405 = 404/405

đáp số : 404/405

\(\dfrac{15}{75}=\dfrac{15:15}{75:15}=\dfrac{1}{5}\\ \dfrac{28}{98}=\dfrac{28:14}{98:14}=\dfrac{2}{7}\\\dfrac{13}{78}=\dfrac{13:13}{78:13}=\dfrac{1}{6}\\ \dfrac{21}{49}=\dfrac{21:7}{49:7}=\dfrac{3}{7}\\ \dfrac{135}{405}=\dfrac{135:135}{405:135}=\dfrac{1}{3}\)

A) 44,6 + (53,5 +63,9) :10

\(=44,6+117,4\div10\)

\(=44,6+11,74\)

\(=56,34\)

B) 675 :45 -135:45

\(=\frac{1}{45}\times\left(675-135\right)\)

\(=\frac{1}{45}\times540\)

\(=12\)

C) 20,4 :24 +246:24

\(=\frac{1}{24}\times\left(20,4+246\right)\)

\(=\frac{1}{24}\times266,4\)

\(=11,1\)

CHÚC BN HỌC TỐT NHA!!!!

56,34

12

30.65

\(\text{ a.135 x 345 + 675 : 135 }\)

\(=46575+5\)

\(=46580\)

\(\text{ b.15 400 : 25 : 4 }\)

\(=616:4\)
\(=154\)

\(\text{ c.1224 : 24 + 1416 : 24}\)

\(=\left(1224+1416\right):24\)

\(=2640:24\)

\(=110\)

học tốt

a.135x345+675:135

=46575+5

=46580

b.15400:25:4

=616:4

=154

c.1224:24+1416:24

=(1224+1416):24

=2640:24

=110

k đúng cho mình nha

câu a thì lấy 15400:25 là được.

a) \(15400:x=25\)

\(\Rightarrow x=15400:25\)

\(\Rightarrow x=616\)

b) \(\left(x+3\right):247=675:135\)

\(\Rightarrow\left(x+3\right):247=5\)

\(\Rightarrow x+3=5\times247\)

\(\Rightarrow x+3=1235\)

\(\Rightarrow x=1235-3\)

\(\Rightarrow x=1232\)

c) \(1224:\left(x-8\right)=24\)

\(\Rightarrow\left(x-8\right)=1224:24\)

\(\Rightarrow x-8=51\)

\(\Rightarrow x=51+8\)

\(\Rightarrow x=59\)

a) 15400 : x = 25

                  x= 15400 : 25

                  x= 616

D/S: 616

b) (x+3) : 247 = 675 : 135

    (x+3) : 247 = 5

    x+3            = 5 x 247

   x+3             = 1235

   x                 = 1235 - 3

    x               =  1230

Đ/S : 1230 

c) 1224 : (x-8)  = 24

                x-8   = 1224:24

                x-8 = 51

                x    = 51+8

                x= 59

Đ/S : 59