I just wanted to know if my answer is acceptable.
Q: S={(x,y,z) E \mathbb{R}^{3} l x^2 + y^2 +z ^2 =0}
Is it a subspace of \mathbb{R}^{3}?
My answer:
It is a subspace if x=0, y =0, z= 0
Let u=(0,0,0) u2=(0,0,0) and k be a scalar
u + u2 = (0,0,0) Closed under addition...