28/x + 0,7 = 28/(x-2)
⇔ 28/x + 7/10 = 28/(x-2)
⇔ 28/x - 28/(x-2) + 7/10 = 0
⇔ [ 28(x-2) - 28x ]/x(x-2) + 7/10 = 0
⇔56/x(x-2) = 7/10
⇔ 7x(x-2)= 560
⇔ 7x² - 14x -560 = 0
⇔ x² - 2x - 80 = 0
⇔ x² -10x +8x -80 = 0
⇔ x(x-10) + 8(x-10) = 0
⇔ (x -10)(+8) = 0
⇔x -10= 0 hoặc x+8=0
⇔ x= 10 hoặc x= -8