Đáp án `+` Giải thích các bước giải`:`

Bài `20`

`a)`

`{( \sqrt{x-2} + \sqrt{y-3} =3   ),( 2.\sqrt{x-2}  - 3\sqrt{y-3} = -4 ):}`

`<=>{( \sqrt{x-2} = 3 - \sqrt{y-3} ),( 2\sqrt{x-2} -3\sqrt{y-3} =-4 ):}`

`<=>{( \sqrt{x-2} =3 -\sqrt{y-3}  ),( 2(3-\sqrt{y-3}) -3\sqrt{y-3} =4 ):}`

`<=>{( \sqrt{x-2} =3 -\sqrt{y-3} ),( y=7):}`

`<=>{(x=3),(y=7):}`

Vậy`...`

`b)`

`{( (3x)/(x+1) + 2/(y+4) =4 ),( (2x)/(x+1) - 5/(y+4) =9 ):}`

`<=>{( 3x . (y+4) + (x+1) .2 = 4xy +4y+16x+16),((2x)/(x+1) - 5/(y+4) =9):}`

`<=>{( 3xy - 4xy -4y = 16x+16-14x -14x -2 ),((2x)/(x+1) - 5/(y+4) =9):}`

`<=>{( (3x-4x-4).y = 16x+16-14x-2 ),((2x)/(x+1) - 5/(y+4) =9):}`

`<=>{(y = (16x +16 - 14x -2)/(3x-4x-4) ),((2x)/(x+1) - 5/(y+4) =9):}`

`<=>{(y = - (2x+14)/(x+4) ),((2x)/(x+1) - 5/(y+4) =9 ):}`

`<=>{( y = - (2x+14)/(x+4) ),( (2x)/(x+1) - 5/( ( - (2x+14)/(x+4) +4  ) ) = 9  ):}`

`<=>{(y = - (2x+14)/(x+4) ),(x=-2):}`

`<=>{(y=-5),(x=-2):}`

Vậy`...`

`c)`

`{( 2 . (x^2 -2x) +\sqrt{y+1} = 0 ),( 3(x^2 -2x) -2 \sqrt{y+1} +7 =0):}`

`<=>{(2x^2 -4x + \sqrt{y+1} =0 ),( 3x^2 -6x - 2\sqrt{y+1} +7 = 0):}`

`<=>{(\sqrt{y+1} = -2x^2 +4x ),(3x^2 -6x-2\sqrt{y+1} +7=0):}`

`<=>{(\sqrt{y+1} = - 2x^2 +4x ),(3x^2 - 6x- 2 (-2x^2 +4x)+7=0):}`

`<=>{(\sqrt{y+1} = -2x^2 +4x. 1 ),(x=1):}`

`<=>{(y=3),(x=1):}`

Vậy`...`

`#`Prodigo`#`

`***Hoidap247`