Под а и б пж быстро нужнл

[latex] \frac{15 x^{3} y^{2} }{14 a^{2} b^{2} } : \frac{20 x^{2} y^{3} }{21 a^{2} b}= \frac{15 x^{3} y^{2} }{14 a^{2} b^{2} }* \frac{21 a^{2} b}{20 x^{2} y^{3} }= \frac{15x}{14b}* \frac{21}{20y}= \frac{9x}{8by} \\ \\ \frac{z+y}{ z^{2} -yz} * \frac{z}{z+y} = \frac{z}{ z^{2}-y } = \frac{z}{z(z-y)} = \frac{1}{z-y} [/latex]