subs x as x+1
so
f(x)+f(x+2)=√2f(x+1)
subs x as x-1 in d orig eqn
f(x-2)+f(x)=√2f(x-1)
add both
f(x-2)+f(x+2)+2f(x)=√2(f(x-1)+f(x+1))
f(x-2)+f(x+2)+2f(x)=2f(x)
f(x-2)+f(x+2)=0
subx x as x+2
f(x)+f(x+4)=0
subs x as x-2
f(x-4)+f(x)=0
subtract...both
f(x+4)=f(x-4)
subs x as x-4
f(x)=f(x-8)