If `n` is even, so that `n=2k` for some integer `k,` then
`n^2=(2k)^2=4k^2,` so `n^2-=0 (mod 4)` if `n` is even.
If `n` is odd, then `n=2j+1` for some integer `j.` In that case,
so `n^2-=1(mod 4)` if `n` is odd. These are the only two possibilities, so the proof is complete.
To prove n^2 is either divisible by 4 or leaves remainder as 1. Natural number are in two set odd and even
let n is even number i.e n=2m ,m is a nutural number.
n^2=(2m)^2=4m^2 ,which is multiple of 4 so is divisible by 4 and remainder will 0.
Let n is odd number i.e. n=2m+1, m is natural no.
first term in above expression is multiple of 4 so divisble by 4 ,and seconder term leaves remainder 1.
This statement is not true for n=1 ,1 is natural no.