1i think that the required proof is not correct.
let the mass of a nutron be 'm' and vel. be 'v'
the velocity of(A+1)m be 'V'.
from con. of momentum-
mv=(A+1)mV
or,V=v/(A+1)
change in KE-
12(A+1)mV2 -12mv2
fractional ch. in KE-
{(A+1)V2 -v2}/v2
putting the value of V we get the result=
-A/(A+1)
