(4х² - 25у²) * ( 1/(2х+5у) + 1/(2х-5у) ) =
(4х² - 25у²) * ( (2х-5у)/(2х+5у)*(2х-5у) + (2х+5у)/(2х-5у)*(2х+5у) ) =
(4х² - 25у²) * ( (2х-5у) + (2х+5у)/(2х-5у)*(2х+5у) ) =
(4х² - 25у²) * ( (2х-5у + 2х+5у)/(2х-5у)*(2х+5у) ) =
(4х² - 25у²) * ( (4х)/(2х-5у)*(2х+5у) ) =
( (2х-5у)*(2х+5у) ) * ( (4х)/(2х-5у)*(2х+5у) ) = 4х