`g)` `(x+4)/(5x-25) . (x-5)/(x^2+ 8x+16)` (ĐK: `x\ne5, x\ne -4)`

`= (x+4)/(5(x-5)) . (x-5)/(x+4)^2`

`= 1/5. 1/(x+4)`

`= 1/(5(x+4))`

`= 1/(5x+20)`

$\text{B1: Tìm ĐKXĐ}$

$\text{B2: Đặt 5 là nhân tử chung}$

$\text{B3: Dùng HĐT a²+ 2ab+ b²= (a+b)²}$

$\text{B4: Rút gọn}$

$\text{B5: Thực hiện phép toán}$

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`h) (a^2- b^2)/(9b^2) : (a+b)/(3b)` (ĐK:` b \ne 0; a\ne -b)`

`= ((a-b)(a+b))/(9b^2) . (3b)/(a+b)`

`= (a-b)/(3b)`

$\text{B1: Tìm ĐKXĐ}$

$\text{B2: Dùng HĐT: a²-b²=(a-b)(a+b)}$

$\text{B3: Rút gọn}$

g) ` [ x + 4 ] / [ 5x - 25 ] . [ x - 5 ] / [ x² + 8x + 16 ] `

` =  [ x + 4 ] / [ 5 ( x - 5 ) ] . [ x - 5 ] / [ ( x + 4 )² ] `

` =  1 / 5 . 1 / [ x + 4 ] `

` = 1/ [ 5 . ( x + 4 ) ] `

h) ` [ a² - b² ] / [ 9b² ] : [ a + b ] / [ 3b ] `

` =  [ ( a - b ) ( a + b ) ] / [ 3b . 3b ] . [ 3b ] / [ a + b ] `

` =  [ a - b ] / [ 3b ] `