a) $\dfrac{1-x^2}{7x^3y}+\dfrac{5x^3y}{7x^3y}+\dfrac{x^2-1}{7x^3y}$ ĐK: $x\neq0,y\neq0$
$=\dfrac{1-x^2+5x^3y+x^2-1}{7x^3y}$
$=\dfrac{5x^3y}{7x^3y}$
$=\dfrac{5}{7}$
b) $\dfrac{8}{9x^2y^3}+\dfrac{7}{18x^3y^2}+\dfrac{x-1}{12xy^3}$ ĐK: $x\neq0,y\neq0$
$=\dfrac{32x+14y+3x^2(x-1)}{36x^3y^3}$
$=\dfrac{32x+14y+3x^3-3x^2}{36x^3y^3}$
c) $\dfrac{5}{2x}+\dfrac{x-3}{x^2-1}+\dfrac{6}{x+1}$ ĐK: $x\neq0,x\neq\pm1$
$=\dfrac{5}{2x}+\dfrac{x-3}{(x-1)(x+1)}+\dfrac{6}{x+1}$
$=\dfrac{5(x-1)(x+1)+2x(x-3)+12x(x-1)}{2x(x-1)(x+1)}$
$=\dfrac{5x^2-5+2x^2-6x+12x^2-12x}{2x(x-1)(x+1)}$
$=\dfrac{19x^2-18x-5}{2x(x-1)(x+1)}$
