Rút gọn phân số : a, (-14).11+14.12/11.21-7.12 ; b, 7/9.10^2-2.10^2.

$a)$ $\dfrac{\left(-14\right)\cdot \:11+14\cdot \:12}{11\cdot \:21-7\cdot \:12}$

$=\dfrac{\left(\left(-14\right)\cdot 11\right)+\left(14\cdot 12\right)}{\left(11\cdot 21\right)-\left(7\cdot 12\right)\:}$

$=\dfrac{-154+168}{231-84}$

$=\dfrac{14}{147}$

$=\dfrac{7\cdot \:2}{7\cdot \:21}$

$=\dfrac{2}{21}$

________________________________________

$b)$ $\dfrac{7}{9}\cdot 10^2-2\cdot 10^2$

$=\left(\dfrac{7}{9}\cdot \:10^2\right)-\left(2\cdot \:10^2\right)$

$=\dfrac{700}{9}-200$

$=-\dfrac{1100}{9}$