Question
The set of real values of $$k$$ for which the equation $$\left( {k + 1} \right){x^2} + 2\left( {k - 1} \right)xy + {y^2} - x + 2y + 3 = 0$$ represents an ellipse is :
A.
$$\left( {0,\,3} \right)$$
B.
$$\left( { - \infty ,\,0} \right)$$
C.
$$\left( {3,\, + \infty } \right)$$
D.
$$\left( { - \infty ,\,\infty } \right)$$
Answer :
$$\left( {0,\,3} \right)$$
Solution :
For ellipse, $$ab - {h^2} > 0\,\,\, \Rightarrow \left( {k + 1} \right).1 - {\left( {k - 1} \right)^2} > 0{\text{ or }}k\left( {k - 3} \right) < 0$$