HI FRIENDS.
may i know some clarification
1.what is body effect and channel length modulation?
2.in channel lenth modulation
how L'=L-ΔL is written as 1/L'≈(1+ΔL/L)L.
There are a lots of materials over net and good books available.
1) Design of Analog CMOS Integrated Circuits, Behzad Razavi
2) Also you can get basic info from Wikipedia
1/L' = (1+ΔL/L)L i.e. L' = ((1+ΔL/L)L)^-1 i.e. inverse
Now ΔL/L is very small. Look for (1+x)^-1 binomial expansion (when |x|<1)over internet. It can be approximated as 1-x+x^2-x^3+ ..... = 1-x (approx)