2019 . 20182018 - 2018 . 20192019

= 2019 . 10001 . 2018 - 2018 . 10001 . 2019

= 10001 ( 2019 . 2018 - 2018 . 2019 )

= 10001 . 0

= 0

bị sai dấu sorry

a)A=\(x^2-4y^2-3x^2+2y^2\)

=\(-2x^2-2y^2\)

thay x=1;y=-1 vào A ta có:

A=\(-2.1^2-2.\left(-1\right)^2\)

=-4

f)=\(\left(\dfrac{3}{4}+\dfrac{1}{2}\right):\left(\dfrac{4}{5}-\dfrac{3}{5}\right)\)

=\(\left(\dfrac{6}{8}+\dfrac{4}{8}\right):\left(\dfrac{4}{5}-\dfrac{3}{5}\right)\)

=\(\dfrac{5}{4}:\dfrac{1}{5}\)

=\(\dfrac{25}{4}\)

=\(\dfrac{3}{11}.\left(\dfrac{4}{3}-\dfrac{7}{4}\right)\)

=\(\dfrac{3}{11}.\left(\dfrac{16}{12}-\dfrac{21}{12}\right)\)

=\(\dfrac{3}{11}.\left(-\dfrac{5}{12}\right)\)

=\(-\dfrac{15}{132}\)

=\(-\dfrac{5}{44}\)