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Đáp án $+$ Giải thik các bước giải

* Rút gọn rồi tính

`a) 78/42 - 4/7`

`78/42 =` $\dfrac{78:6}{42:6}$ `= 13/7`

`=13/7 - 4/7`

`=` $\dfrac{13-4}{7}$

`= 9/7`

`b) 100/110 - 56/88`

`100/110 =` $\dfrac{100:100}{110:10}$ `= 10/11`

`56/88 =` $\dfrac{56:8}{88:8}$ `= 7/11`

`= 10/11 - 7/11`

`=` $\dfrac{10-7}{11}$

`= 3/11`

`c) 75/35 - 11/7 + 21/49`

`75/35 =` $\dfrac{75:5}{35:5}$ `= 15/7`

`21/49 =` $\dfrac{21:7}{49:7}$ `= 3/7`

`= 15/7 - 11/7 + 3/7`

`=` $\dfrac{15-11+3}{7}$

`= 7/7`

`= 1`

`d) 99/72 - 15/40 - 1212/1616`

`99/7``2 =` $\dfrac{99:9}{72:9}$ `= 11/8`

`15/40 =` $\dfrac{15:5}{40:3}$ `= 3/8`

`1212/1616 =` $\dfrac{1212:202}{1616:202}$ `= 6/8`

`= 11/8 - 3/8 - 6/8`

`=` $\dfrac{11-3-6}{8}$

`= 2/8`

`= 1/4`

* Tìm $x$

`a) 24/36 = x/12`

`24/36 =` $\dfrac{24:3}{36:3}$ `= 8/12`

`8/12 = x/12`

`x = 8`

Vậy `x = 8`

`b) 14/56 = 1/x`

`14/56 =` $\dfrac{14:14}{56:14}$ `= 1/4`

`1/4 = 1/x`

`x = 4`

Vậy `x = 4`

`c) 9/24 = x/56`

`x` `xx` `24 = 9` `xx` $56$

`x` `xx` `24 = 504`

`x = 504 : 24`

`x = 21`

Vậy `x = 21`

Hoặc

`9/24 = x/56`

`9/24 =` $\dfrac{9:3}{24:3}$ `= $\dfrac{3}{8}$

`3/8 = x/56`

`3/8 = ( 3xx7 )/( 8xx7 ) = 21/56`

`21/56 = x/56`

`x = 21`

Vậy `x = 21`

Đáp án + Giải thích các bước giải:

Bài 1:

a) `78/42 - 4/7`

`= 13/7 - 4/7 = 9/7`

b) `100/110 - 56/88`

`= 10/11 - 7/11`

`= 3/11`

c) `75/35 - 11/7 + 21/49`

`= 15/7 - 11/7 + 3/7`

`= 4/7 + 3/7 = 7/7 = 1`

d) `99/72 - 15/40 - 1212/1616`

`= 11/8 - 3/8 - 12/16`

`= 11/8 - 3/8 - 6/8`

`= 8/8 - 6/8 = 2/8 = 1/4`

Bài 2:

Tìm x:

a) `24/36 = x/12`

    `8/12 = x/12`

Vậy `x = 8`

b) `14/56 = 1/x`

    `1/4 = 1/x`

Vậy `x = 4`

c) `9/24 = x/56`

    `3/8 = x/56`

    `21/56= x/56`

Vậy `x = 21`