v4/7 – x = 1/7
4/7 – x = (1/7)2
4/7 – x = 1/49
28/49 – x = 1/49
x = 27/49
åñëè òàê
v4/(7 – x) = 1/7
x= – 189

è åùå ìîæíî ïîíÿòü òàê
v4/7 – x = v1/7
x = 3/7