Câu 1: $(x+3)(x²+3x-5)$
$=x(x²+3x-5)+3(x²+3x-5)$
$=x³+3x²-5x+3x²+9x-15$
$=x³+6x²+4x-15$
Câu 2: $(12x^4y^3-20x^5y^2-28x^6y^4):(-2x^4y^2)$
$=12x^4y^3:(-2x^4y^2)-20x^5y^2:(-2x^4y^2)-28x^6y^4:(-2x^4y^2)$
$=-6y+10x+14x^2y^2$
Câu 3: $C$
$25x²-9=(5x-3)(5x+3)$
Câu 4: $(3x+1)²-(x+1)²$
$=(3x+1+x+1)(3x+1-x-1)$
$=(4x+2)2x$
$=8x²+4x$
Câu 5: Thay $x=6,y=-4,z=45$
$=6²-2.6.(-4)-4.45²+(-4)²$
$=-8000$
Câu 6:
$\dfrac{A}{2x-1}=\dfrac{6x²+3x}{4x²-1}$
$↔\dfrac{A}{2x-1}=\dfrac{3x(2x+1)}{(2x-1)(2x+1)}$
$↔\dfrac{A}{2x-1}=\dfrac{3x}{2x-1}$
$↔A=3x$
Câu 7: $\begin{cases}2x(x+3)\\3x(x+1)\end{cases}→6x(x+1)(x+3)$
câu 8: $\dfrac{3x+1}{x²-3x+1}+\dfrac{x²-6x}{x²-3x+1}$
$=\dfrac{3x+1+x²-6x}{x²-3x+1}$
$=\dfrac{x²-3x+1}{x²-3x+1}=1$
Câu 9: $\dfrac{x+1}{x+2}:(\dfrac{x+2}{x+3}.\dfrac{x+3}{x+1})$
$=\dfrac{x+1}{x+2}:\dfrac{x+2}{x+1}$
$=\dfrac{x+1}{x+2}.\dfrac{x+1}{x+2}$
$=\dfrac{(x+1)²}{(x+2)²}$
Câu 10: ĐKXĐ: $x²-4y²\ne 0$
$↔x²\ne 4y²$
$↔x\ne ±2y$
