s_n = u + a/2(2n - 1)

s_n-1 = u + a/2(2n - 3)

 

s_n +  s_n-1  = 2u + 2an - 2a

displacement of particle in last 2 secs = 2(u + an - a)

 

v = u + at

v = u + 2a

u = v - 2a

 

therefore, 2(u + an - a) = 2(v - 3a + an)

 

therefore, displacement of particle in last 2 secs = 2(v - 3a + an)