Question

已知x分之1-y分之1=3,求（x-xy-y）分之（5x+xy-5y）的值

Answer 1

1/x-1/y=3

(y-x)/xy=3

y-x=3xy

x-y=-3xy

(5x+xy-5y)/(x-xy-y)

=[5(x-y)+xy]/[(x-y)-xy]

=[5*(-3xy)+xy]/[(-3xy)-xy]

=(-14xy)/(-4xy)

=7/2

=4