?

m2c2=m?2c2/(c2-v2)

∵cv∴c2-v2≠0

?

m2c2(c2-v2)=m?2c2

?

d[m2c2(c2-v2)]/dv=d(m?2c2)/dv

∵m?，c均为常数;

∴d(m?2c2)/dv=0

?

(c2-v2)d(m2c2)/dv+m2c2d(c2-v2)/dv=0

?