mình mới học lớp 5 thôi !

Thông cảm cho mình nhé Do uyen Linh !

uuttqquuậậyy gửi từng bài thì có mà hết lượt gửi câu hỏi à

a) \(xy+3x-7y=21\)

\(\Leftrightarrow x\left(y+3\right)-7y-21=0\)

\(\Leftrightarrow x\left(y+3\right)-7\left(y+3\right)=0\)

\(\Leftrightarrow\left(x-7\right)\left(y+3\right)=0\)

\(\Leftrightarrow\hept{\begin{cases}x-7=0\\y+3=0\end{cases}}\Leftrightarrow\hept{\begin{cases}x=7\\y=-3\end{cases}}\)

Vậy x = 7 và y = -3

b) \(xy+3x-2y=11\)

\(\Leftrightarrow x\left(y+3\right)-2y-6=5\)

\(\Leftrightarrow x\left(y+3\right)-2\left(y+3\right)=5\)

\(\Leftrightarrow\left(x-2\right)\left(y+3\right)=5=1.5=5.1=\left(-1\right).\left(-5\right)=\left(-5\right).\left(-1\right)\)

Lập bảng:

Vậy \(\left(x,y\right)\in\left\{\left(3,2\right);\left(7,-2\right);\left(1,-8\right);\left(-3,-3\right)\right\}\)

a

\(xy+3x-7y-21\\ =\left(xy+3x\right)-\left(7y+21\right)\\ =x\left(y+3\right)-7\left(y+3\right)\\ =\left(y+3\right)\left(x-7\right)\)

b

\(2xy-15-6x+5y\\ =\left(2xy-6x\right)-\left(15-5y\right)\\ =2x\left(y-3\right)-5\left(3-y\right)\\ =2x\left(y-3\right)+5\left(y-3\right)\\ =\left(y-3\right)\left(2x+5\right)\)

c Đề phải là \(\left(2x^2y+2xy^2-x-y\right)\) mới phân tích được: )

\(=2xy\left(x+y\right)-\left(x+y\right)\\ =\left(x+y\right)\left(2xy-1\right)\)

d

\(7x^3y-3xyz-21x^2+9z\\ =\left(7x^3y-21x^2\right)-\left(3xyz-9z\right)\\ =7x^2\left(xy-3\right)-3z\left(xy-3\right)\\ =\left(xy-3\right)\left(7x^2-3z\right)\)

e

\(4x^2-2x-y^2-y\\ =\left(2x\right)^2-y^2-\left(2x+y\right)\\ =\left(2x-y\right)\left(2x+y\right)-\left(2x+y\right)\\ =\left(2x+y\right)\left(2x-y-1\right)\)

f

\(9x^2-25y^2-6x+10y\\ =\left(3x\right)^2-\left(5y\right)^2-\left(6x-10y\right)\\ =\left(3x-5y\right)\left(3x+5y\right)-2\left(3x-5y\right)\\ =\left(3x-5y\right)\left(3x+5y-2\right)\)

a: =x(y+3)-7(y+3)

=(y+3)(x-7)

b: \(=2xy-6x+5y-15\)

=2x(y-3)+5(y-3)

=(y-3)(2x+5)

c: \(=2xy\left(x+y\right)-\left(x+y\right)=\left(x+y\right)\left(2xy-1\right)\)

d: \(=xy\left(7x^2-3z\right)-3\left(7x^2-3z\right)\)

=(7x^2-3z)(xy-3)

e: =4x^2-y^2-2x-y

=(2x-y)(2x+y)-(2x+y)

=(2x+y)(2x-y-1)

f: =(3x-5y)(3x+5y)-2(3x-5y)

=(3x-5y)(3x+5y-2)

A)

Ta có : xy+3x-7y=21 ==> x(y+3)-7y-21=0

                               ==> x(y+3)-7y-7.3=0 ==> x(y+3)-7(y+3) =0 ==> (x-7)(y+3)=0

==> x-7 hoặc y+3 bằng 0. 

==> x=7 ; y=3

Sau đó thay x=7 , y=3 là ra x,y tương ứng