maths problem

Yes it is.
Because:
f(x+y)
=(x+y) f times

for example
2(x+y)
=x+y+x+y
=2x+2y.

You can justify your answer like that

係
其實你只要代d 數入去就知係定唔係
e家設
f=2
x=3
y=4
2x(3+4)=2x3+2x4
14=14
所以e條式係岩既

NO
assume f(x)=x^2
when x=2,y=3
L.H.S=5^2
=25
R.H.S=2^2+3^2
=4+9
=13
because L.H.S is not equal to R.H.S
therefore,the answer is NO

Yes is it.(x+y)=f(x)+f(y)

yes.f(x+y)=(f)(x)+(f)(y)