Which of the following is an inner product on R2\mathbb{R}^2R2?
⟨u,v⟩=u1v2+u2v1\langle u, v \rangle = u_1v_2 + u_2v_1⟨u,v⟩=u1v2+u2v1
⟨u,v⟩=u1v1−u2v2\langle u, v \rangle = u_1v_1 - u_2v_2⟨u,v⟩=u1v1−u2v2
⟨u,v⟩=u1v1+u2v2\langle u, v \rangle = u_1v_1 + u_2v_2⟨u,v⟩=u1v1+u2v2
⟨u,v⟩=u1+v1\langle u, v \rangle = u_1 + v_1⟨u,v⟩=u1+v1