Which values of kkk ensure that the system x+y=kx+y=kx+y=k and x2+y2=1x^2+y^2=1x2+y2=1 has real solutions?
k∈[0,2]k \in [0, 2]k∈[0,2]
k∈[−1,1]k \in [-1, 1]k∈[−1,1]
k>0k > 0k>0
k∈[−2,2]k \in [-\sqrt{2}, \sqrt{2}]k∈[−2,2]