Question

(x的-1次方+y的-1次方)÷(x的-2次方-y的-2次方)

Answer 1

(x的-1次方+y的-1次方)÷(x的-2次方-y的-2次方)

=(1/x+1/y)÷(1/x+1/y)(1/x-1/y)

=1/(1/x-1/y)

=xy/(y-x);