С^2+m^2/c^3-m^2 + m/c+m + m/m-c

= c²+m² / (c-m)(c+m) + m(c-m) / (c-m)(c+m) - m(c+m) / (c-m)(c+m) = c²+m²+cm-m²-cm-m² / c²-m² = c²-m² / c²-m² = 1