Question

(x/9 - y/5) . (x/4 + y/5)

Answer 1

Basta fazer contas de multiplicação, um com o outro, veja:

$(\frac{x}{9}-\frac{y}{5}) . (\frac{x}{4} + \frac{y}{5})\\\\\\ \frac{x}{9}.\frac{x}{4}+\frac{x}{9}.\frac{y}{5}-\frac{y}{5}.\frac{x}{4}-\frac{y}{5}.\frac{y}{5}\\\\\\ \frac{x^2}{36}+\frac{xy}{45}-\frac{xy}{20}-\frac{y^2}{25}$

$mmc\ de\ 36,45, 20\ e\ 25\ e : 900\\\\\ 25x^2+20xy-45xy-36y^2\\\\\ \boxed{25x^2-25xy-36y^2}$

Answer 2

(x/9 - y/5) . (x/4 + y/5)

  x^2 + xy  - xy   -   y^2
  36     45   20      25

x^2 + 20xy-45xy  - y^2
36           900          25

x^2  - 5xy  - y^2
36     900    25