\(\left(2x-5\right)\left(3x+1\right)-6x\left(x-3\right)\)

\(=6x^2+2x-15x-5-6x^2+18x=5x-5\)

`(2x-5)(3x+1)-6x (x-3)`

`= 6x^2 +2x-15x-5 - 6x^2 + 18x`

`= (6x^2 - 6x^2)+(2x-15x+18x)-5`

`= 5x-5`

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

\(=x^2y^2+xy-x^2y+xy^2+xy+1-x+y+x^2y+x-x^2+xy-xy^2-y+xy-y^2\)

\(=x^2y^2+2xy-x^2-y^2+1\)

ta có 

\(A+B+C+D=360^0\text{ nên }C+D=360^0-A-B\)

bài 1. ta sẽ có : \(\hept{\begin{cases}C+D=360^0-50^0-50^0=260^0\\C=3D\end{cases}\Leftrightarrow\hept{\begin{cases}C=195^0\\D=65^0\end{cases}}}\)

bài 2. ta sẽ có : \(\hept{\begin{cases}C+D=360^0-50^0-70^0=240^0\\C=2D\end{cases}\Leftrightarrow\hept{\begin{cases}C=160^0\\D=80^0\end{cases}}}\)

bài 1. ta sẽ có : \(\hept{\begin{cases}C+D=360^0-60^0-60^0=240^0\\C-D=20^0\end{cases}\Leftrightarrow\hept{\begin{cases}C=130^0\\D=110^0\end{cases}}}\)

\(8x\left(x-2\right)-3\left(x^2-4x-5\right)-5x^2\)

\(=8x^2-16x-3x^2+12x+15-5x^2\)

\(=15-4x\)

`8x(x-2) -3 (x^2 -4x-5)-5x^2`

`= 8x^2 - 16x - 3x^2 +12x+15 - 5x^2`

`= (8x^2 - 3x^2 - 5x^2)+(-16x +12x)+15`

`= -4x +15`

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

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

LÀM TIẾP NHA

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

\(\left(-2a+3\right)\left(-2+3a\right)=4a-6a^2-6+9a=6a^2+13a-6\)

`(-2a+3) (-2+3a)`

`= -2a (-2+3a)+3 (-2+3a)`

`= 4a - 6a^2 -6 + 9a`

`=  -6a^2 + 13a - 6`